Feb. 28, 2025

(Morgan Stanley Mortgage Securities Trust - Classes A, L, I, C and R6) | (Mortgage Securities Trust)

Investment Objective

Morgan Stanley Mortgage Securities Trust (the “Fund”) seeks a high level of current income.

Fees and Expenses

The table below describes the fees and expenses that you may pay if you buy, hold and sell shares of the Fund. You may pay fees other than the fees and expenses of the Fund, such as brokerage commissions and other fees charged by financial intermediaries, which are not reflected in the tables and examples below.

For purchases of Class A shares, you may qualify for a sales charge discount if the cumulative net asset value per share (“NAV”) of Class A shares of the Fund being purchased in a single transaction, together with the NAV of any shares of the Fund and/or certain other Morgan Stanley Funds already held in Related Accounts (as defined in the section of the Prospectus entitled “Shareholder Information—Share Class Arrangements”) as of the date of the transaction, amounts to $100,000 or more.  More information about this combined purchase discount and other discounts is available from your authorized financial intermediary, on page 35 of the Prospectus in the section entitled “Shareholder Information—Share Class Arrangements” and in Appendix A attached to the Prospectus. In addition, Appendix A attached to the Prospectus contains more information regarding financial intermediary specific sales charge waivers and discounts.

Class I shares may be available on brokerage platforms of firms that have agreements with the Fund’s principal underwriter permitting such firms to (i) offer Class I shares solely when acting as an agent for the investor and (ii) impose on an investor transacting in Class I shares through such platforms a commission and/or other forms of compensation to the broker. Shares of the Fund are available in other share classes that have different fees and expenses.

Shareholder Fees (fees paid directly from your investment)

Annual Fund Operating Expenses (expenses that you pay each year as a percentage of the value of your investment)

Example

The example below is intended to help you compare the cost of investing in the Fund with the cost of investing in other mutual funds.

The example assumes that you invest $10,000 in the Fund, your investment has a 5% return each year and that the Fund’s operating expenses remain the same (except that the example incorporates the fee waiver and/or expense reimbursement arrangement for only the first year). After eight years, Class C shares of the Fund generally will convert automatically to Class A shares of the Fund. The example for Class C shares reflects the conversion to Class A shares after eight years. Please refer to the section of the Prospectus

entitled “Shareholder Information—Conversion Features” for more information.  Although your actual costs may be higher or lower, based on these assumptions your costs would be:

If You SOLD Your Shares

If You HELD Your Shares

Portfolio Turnover

The Fund pays transaction costs, such as commissions, when it buys and sells securities (or “turns over” its portfolio). A higher portfolio turnover rate may indicate higher transaction costs and may result in higher taxes when Fund shares are held in a taxable account. These costs, which are not reflected in Total Annual Fund Operating Expenses or in the Example, affect the Fund’s performance. During the most recent fiscal year, the Fund’s portfolio turnover rate was 383%  of the average value of its portfolio.

Principal Investment Strategies

The Fund normally invests at least 80% of its assets in mortgage-related securities. This policy may be changed without shareholder approval; however, you would be notified upon 60 days’ notice in writing of any changes.  These mortgage-related securities may include mortgage-backed securities such as mortgage pass-through securities, collateralized mortgage obligations (“CMOs”), stripped mortgage-backed securities (“SMBS”), commercial mortgage-backed securities (“CMBS”) and inverse floating rate obligations (“inverse floaters”). The mortgage-backed securities in which the Fund invests may be issued or guaranteed by the U.S. Government, its agencies or instrumentalities or may be offered by non-governmental issuers, such as commercial banks, savings and loan institutions, private mortgage insurance companies, mortgage bankers and other secondary market issuers. The Fund is not limited as to the maturities (when a debt security provides its final payment) or types of mortgage-backed securities in which it may invest.

In making investment decisions, the Adviser considers economic developments, interest rate levels and other factors. To identify attractive sectors and securities, the Adviser employs both top-down and bottom-up analyses. In addition, the Adviser combines quantitative and fundamental methodologies to limit the Fund’s investment universe from which potential investments are then selected. In a securitized strategy, such as the strategy employed by the Fund, a majority of the Adviser’s investment process is security selection related.

When deemed by the investment adviser to be relevant to its evaluation of creditworthiness and when applicable information is available, the investment adviser considers environmental, social and/or governance issues (referred to as ESG) which may impact the prospects of an issuer (or obligor) or financial performance of an obligation. When considered, one or more ESG issues are taken into account alongside other factors in the investment decision-making process and are not the sole determinant of whether an investment can be made or will remain in the Fund’s portfolio.

The Fund may invest up to 50% of its net assets in high yield securities (commonly referred to as “junk bonds”). High yield securities are fixed-income securities rated by one or more rating agencies below Baa3 by Moody’s Investors Service, Inc. (“Moody’s”), below BBB- by S&P Global Ratings Group, a division of S&P Global Inc. (“S&P”), below BBB- by Fitch Ratings, Inc. (“Fitch”), or the equivalent by another nationally recognized statistical rating organization (“NRSRO”), or if unrated, considered by the Adviser to be of equivalent quality.

One type of mortgage-backed security in which the Fund may invest is a mortgage pass-through security, which represents a participation interest in a pool of residential mortgage loans originated by U.S. governmental or private lenders such as banks. Mortgage pass-through securities provide for monthly payments that are a “pass-through” of the monthly interest and principal payments made by the individual borrowers on the pooled mortgage loans. CMOs are debt obligations collateralized by mortgage loans or mortgage pass-through securities (collectively “Mortgage Assets”). CMOs are issued in multiple classes and each class has a fixed or floating rate and a stated maturity or final distribution date. Certain classes will have more predictable cash flows than others.

The Fund may invest in any class of CMO. SMBS are derivative multi-class mortgage-backed securities. A common type of stripped mortgage-backed security will have one class receiving some of the interest and most of the principal from the Mortgage Assets, while the other class receives most of the interest and the remainder of the principal. In the most extreme case, one class will receive all of the interest (the interest-only or “IO” class), while the other class will receive all of the principal (the principal-only or “PO” class). CMBS are generally multi-class or pass-through securities backed by a mortgage loan or a pool of mortgage loans secured by commercial property, such as industrial and warehouse properties, office buildings, retail space and shopping malls, multifamily properties and cooperative apartments. Inverse floaters are obligations which pay interest at rates that vary inversely with changes in market rates of interest. Because the interest rate paid to holders of such obligations is generally determined by subtracting a variable or floating rate from a predetermined amount, the interest rate paid to holders of such obligations will decrease as such variable or floating rate increases and increase as such variable or floating rate decreases. In addition, the Fund may invest in to-be-announced pass-through mortgage securities, which settle on a delayed delivery basis (“TBAs”).

The Fund also may invest in other U.S. government securities, including, but not limited to, U.S. Treasury bills, notes and bonds, securities (including mortgage-backed securities) issued by agencies or instrumentalities of the U.S. Government which may or may not be backed by the full faith and credit of the United States, and securities issued by agencies or instrumentalities which are backed solely by the credit of the  issuing agency or instrumentality.

The Fund may also invest in asset-backed securities and restricted and illiquid securities.

In addition, the Fund may invest up to 20% of its net assets in foreign securities, including U.S. dollar-denominated securities issued in the U.S. capital markets by foreign issuers, some of which are commonly known as “Yankee Bonds” and non-U.S. dollar-denominated securities, including Eurobonds.

The Fund may, but it is not required to, use derivatives and similar instruments for a variety of purposes, including hedging, risk management, portfolio management or to earn income. The Fund’s use of derivatives may involve the purchase and sale of derivative instruments such as futures, options, swaps and other similar instruments and techniques. The Fund may utilize foreign currency forward exchange contracts, which are also derivatives, in connection with its investments in foreign securities. These derivative instruments will be counted toward the Fund’s 80% policy discussed above to the extent they have economic characteristics similar to the securities included within that policy.

Principal Risks

The bar chart and table below provide some indication of the risks of investing in the Fund by showing changes in the Fund’s Class A shares’ performance from year-to-year and by showing how the Fund’s average annual returns for the past one, five and 10 year periods and since inception compare with those of a broad measure of market performance and one or more additional indexes. The Fund’s primary benchmark index was changed from the Bloomberg U.S. Mortgage Backed Securities (MBS) Index to the Bloomberg U.S. Universal Index effective May 1, 2024 to comply with the regulation that requires the Fund’s primary benchmark to represent the overall applicable market. The additional index(es) in the table provides a means to compare the Fund’s average annual returns to a benchmark that the Adviser believes is representative of the Fund’s investment universe.  The  performance of the other classes, which is shown in the table  below, will differ because the classes have different ongoing  fees. The performance information in the bar chart does not  reflect the deduction of sales charges; if these amounts were  reflected, returns would be less than shown. The Fund’s returns  in the table include the maximum applicable sales charge for  Class A and Class C and assume you sold your shares at the end of each  period (unless otherwise noted). The Fund’s past performance, before and after taxes, is not necessarily an indication of how  the Fund will perform in the future. Updated performance  information is available online at www.morganstanley.com/im  or by calling toll-free 1-800-869-6397.

Annual Total Returns—Calendar Years

High Quarter	12/31/23	6.18%
Low Quarter	03/31/20	-6.76%

Average Annual Total Returns1(for the calendar periods ended December 31, 2024)

^[1]

The after-tax returns shown in the table above are calculated using the historical highest individual federal marginal income tax rates during the period shown and do not reflect the impact of state and local taxes. After-tax returns for the Fund’s other classes will vary from the Class A shares’ returns. Actual after-tax returns depend on an investor’s tax situation and may differ from those shown, and after-tax returns are not relevant to investors who hold their Fund shares through tax-deferred arrangements, such as 401(k) plans or individual retirement accounts (“IRAs”). After-tax returns may be higher than before-tax returns due to foreign tax credits and/or an assumed benefit from capital losses that would have been realized had Fund shares been sold at the end of the relevant periods, as applicable.

(Morgan Stanley Mortgage Securities Trust - Classes A, L, I, C and R6) | (Mortgage Securities Trust) | Risk Lose Money [Member]

There is no assurance that the Fund will achieve its investment objective, and you can lose money investing in this Fund.

(Morgan Stanley Mortgage Securities Trust - Classes A, L, I, C and R6) | (Mortgage Securities Trust) | Risk Not Insured Depository Institution [Member]

Shares of the Fund are not bank deposits and are not guaranteed or insured by the Federal Deposit Insurance Corporation or any other government agency.

(Morgan Stanley Mortgage Securities Trust - Classes A, L, I, C and R6) | (Mortgage Securities Trust) | Credit and Interest Rate Risk [Member]

• Credit and Interest Rate Risk. Credit risk refers to the possibility that the issuer or guarantor of a security, or counterparty to a transaction, will be unable or unwilling or perceived to be unable or unwilling to make interest payments and/or repay the principal on its debt or otherwise honor its obligations, including the risk of default. In such instances, the value of the Fund could decline and the Fund could lose money. If an issuer’s, guarantor’s or counterparty’s financial condition worsens, the credit quality of the issuer, guarantor or counterparty may deteriorate. Credit ratings may not be an accurate assessment of financial condition, liquidity or credit risk. Although credit ratings may not accurately reflect the true credit risk of an instrument, a change in the credit rating of an instrument or an issuer, guarantor or counterparty, or the market’s perception of the creditworthiness of an instrument or issuer, guarantor or counterparty can have a rapid, adverse effect on the instrument’s value and liquidity and make it more difficult for the Fund to sell at an advantageous price or time. Interest rate risk refers to the decline in the value of a fixed-income security resulting from changes in the general level of interest rates. A wide variety of market and economic factors can cause interest rates to rise or fall, including central bank monetary policy, rising inflation, disinflation or deflation, and changes in general economic conditions. When the general level of interest rates goes up, the prices of most fixed-income securities go down. When the general level of interest rates goes down, the prices of most fixed-income securities go up but the yield or income from new issuances of fixed-income securities generally decreases. Securities with longer durations will generally be more sensitive to changes in interest rates than securities with shorter durations. Fluctuations in interest rates may also affect the liquidity of and income generated by fixed-income instruments held by the Fund.  The Fund may invest in variable and floating rate loans and other variable and floating rate securities. Although these instruments are generally less sensitive to interest rate changes than fixed rate instruments, the value of variable and floating rate loans and other securities may decline if their interest rates do not rise as quickly, or as much, as general interest rates.  The Fund may face a heightened level of interest rate risk in times of monetary policy change and/or uncertainty, such as when the Federal Reserve Board adjusts a quantitative easing program and/or changes rates. Changing interest rates may have unpredictable effects on the markets and may detract from Fund performance. A changing interest rate environment increases certain risks, including the potential for periods of market volatility, increased redemptions, shortened  durations (i.e., prepayment risk) and extended durations (i.e., extension risk).

(Morgan Stanley Mortgage Securities Trust - Classes A, L, I, C and R6) | (Mortgage Securities Trust) | Fixed-Income Securities [Member]

• Fixed-Income Securities. Fixed-income securities are subject to the risk of the issuer’s inability to meet principal and interest payments on its obligations (i.e., credit risk) and are subject to price volatility resulting from, among other things, interest rate sensitivity (i.e., interest rate risk), market perception of the creditworthiness of the issuer and general market liquidity (i.e., market risk). The Fund may face a heightened level of interest rate risk in times of monetary policy change and/or uncertainty, such as when the Federal Reserve Board adjusts a quantitative easing program and/or changes rates. A changing interest rate environment increases certain risks, including the potential for periods of volatility, increased redemptions, shortened durations (i.e., prepayment risk) and extended durations (i.e., extension risk). The Fund is not limited as to the maturities (when a debt security provides its final payment) or durations  (measure of interest rate sensitivity) of the securities in which it may invest.  Securities with longer durations are likely to be more sensitive to changes in interest rates, generally making them more volatile than securities with shorter durations. Lower rated fixed-income securities have greater volatility because there is less certainty that principal and interest payments will be made as scheduled. The Fund may be subject to certain liquidity risks that may result from the lack of an active market and the reduced number and capacity of traditional market participants to make a market in fixed-income securities. To the extent that the Fund invests in convertible securities, and the convertible security’s investment value is greater than its conversion value, its price will be likely to increase when interest rates fall and decrease when interest rates rise. If the conversion value exceeds the investment value, the price of the convertible security will tend to fluctuate directly with the price of the underlying security.  

(Morgan Stanley Mortgage Securities Trust - Classes A, L, I, C and R6) | (Mortgage Securities Trust) | Mortgage-Backed Securities [Member]

• Mortgage-Backed Securities. Because the Fund concentrates its investments in the mortgage-backed securities industry, the Fund has greater exposure to the potential adverse economic, regulatory, political and other changes affecting such industry. Mortgage-backed securities entail prepayment risk, which generally increases during a period of falling interest rates. Rising interest rates tend to discourage refinancings, with the result that the average life and volatility of mortgage-backed securities will increase and market price will decrease. Rates of prepayment, faster or slower than expected by the Adviser, could reduce the Fund’s yield, increase the volatility of the Fund and/or cause a decline in NAV per share. Mortgage-backed securities are also subject to extension risk, which is the risk that rising interest rates could cause mortgages or other obligations underlying the securities to be prepaid more slowly than expected, thereby lengthening the duration of such securities, increasing their sensitivity to interest rate changes and causing their prices to decline. Certain mortgage-backed securities may be more volatile and less liquid than other traditional types of debt securities. In addition, mortgage-backed securities are subject to credit risk. The Fund may invest in non-agency mortgage-backed securities offered by non-governmental issuers, such as commercial banks, savings and loan institutions, private mortgage insurance companies, mortgage bankers and other secondary market issuers. Non-agency mortgage-backed securities are not subject to the same underwriting requirements for the underlying mortgages that are applicable to those mortgage-backed securities that have a government or government-sponsored entity guarantee. As a result, the mortgage loans underlying non-agency mortgage-backed securities may, and frequently do, have less favorable collateral, credit risk or other underwriting characteristics than government or government-sponsored mortgage-backed securities and have wider variances in a number of terms including interest rate, term, size, purpose and borrower characteristics. An unexpectedly high rate of defaults on the mortgages held by a mortgage pool may adversely affect the value of a mortgage-backed security and could result in losses to the Fund. The risk of such defaults is generally higher in the case of mortgage pools that include subprime mortgages. Furthermore, mortgage-backed securities may be subject to risks associated with the assets underlying those securities, such as a decline in value. The Fund may invest a substantial portion of its assets in non-agency mortgage-backed securities rated below investment grade, which are commonly known as “junk bonds” or “high yield/high-risk securities.” The Fund’s investments in high-yield securities pose significant risks. In addition, the Fund may invest in to-be-announced pass-through mortgage securities, which settle on a delayed delivery basis (“TBAs”). Investments in mortgage-backed securities may give rise to a form of leverage (indebtedness) and may cause the Fund’s portfolio turnover rate to appear higher. Leverage may cause the Fund to be more volatile than if the Fund had not been leveraged. The risks associated with mortgage-backed securities typically become elevated during periods of distressed economic, market, health and labor conditions. In particular, increased levels of unemployment, delays and delinquencies in payments of mortgage and rent obligations, and uncertainty regarding the effects and extent of government intervention with respect to mortgage payments and other economic matters may adversely affect the Fund’s investments in mortgage-backed securities. In addition, commercial mortgage-backed securities are also subject to risks associated with reduced demand for commercial and office space, tightening lending standards and increased interest and lending rates, and other developments adverse to the commercial real estate market.

(Morgan Stanley Mortgage Securities Trust - Classes A, L, I, C and R6) | (Mortgage Securities Trust) | Collateralized Mortgage Obligations [Member]

• Collateralized Mortgage Obligations. CMOs are comprised of various tranches, the expected cash flows of which have varying degrees of predictability as compared with the underlying mortgage loans or mortgage pass-through entities. The less predictable the cash flow, the higher the yield and the greater the risk. In addition, if the collateral securing CMOs or any third-party guarantees is insufficient to make payments, the Fund could sustain a loss. Like other mortgage backed-securities, some CMOs are subject to credit risk. The Fund invests in both agency and non-agency CMOs. Many agency CMOs do not have credit risk as they are government guaranteed.

(Morgan Stanley Mortgage Securities Trust - Classes A, L, I, C and R6) | (Mortgage Securities Trust) | Stripped Mortgage-Backed Securities [Member]

• Stripped Mortgage-Backed Securities. Investments in each class of SMBS are extremely sensitive to changes in interest rates. The interest-only or “IO” class tends to decrease in value substantially if interest rates decline and prepayment rates become more rapid. The principal-only or “PO” class tends to decrease in value substantially if interest rates increase and the rate of prepayment decreases. If the Fund invests in SMBS and interest rates move in a manner not anticipated by Fund management, it is possible

that the Fund could lose all or substantially all of its investment. Additionally, some SMBS entail credit risk. The Fund invests in both Agency and Non-Agency bonds. Many Agency bonds do not have credit risk as they are government guaranteed.

(Morgan Stanley Mortgage Securities Trust - Classes A, L, I, C and R6) | (Mortgage Securities Trust) | Commercial Mortgage-Backed Securities [Member]

• Commercial Mortgage-Backed Securities. CMBS are subject to credit risk and prepayment risk. Although prepayment risk is present, it is of a lesser degree in the CMBS market than in the residential mortgage market; commercial real estate property loans often contain provisions which substantially reduce the likelihood that such securities will be prepaid (e.g., significant prepayment penalties on loans and, in some cases, prohibition on principal payments for several years following origination).

(Morgan Stanley Mortgage Securities Trust - Classes A, L, I, C and R6) | (Mortgage Securities Trust) | When-Issued Securities, Delayed Delivery Securities, TBAs and Forward Commitments [Member]

• When-Issued Securities, Delayed Delivery Securities, TBAs and Forward Commitments.  The Fund may purchase or sell securities that it is entitled to receive on a when-issued, delayed delivery or through a forward commitment basis. For example, the Fund may invest in TBAs, which settle on a delayed delivery basis. These investments may result in a form of leverage and may increase volatility in the Fund’s share price. In a TBA transaction, the seller agrees to deliver the MBS for an agreed upon price on an agreed upon future date, but makes no guarantee as to which or how many securities are to be delivered. Accordingly, the Fund’s investments in TBAs are subject to risks such as failure of the counterparty to perform its obligation to deliver the security, the characteristics of a security delivered to the Fund may be less favorable than expected and the security the Fund buys will lose value prior to its delivery. The Fund’s purchase of other securities on a when-issued, delayed delivery or through a forward commitment basis are subject to similar risks. When the Fund has sold a security on a when-issued, delayed delivery, or forward commitment basis, the Fund does not benefit if the value of the security appreciates above the sale price during the commitment period and the Fund is subject to failure of the counterparty to pay for the securities.

(Morgan Stanley Mortgage Securities Trust - Classes A, L, I, C and R6) | (Mortgage Securities Trust) | Inverse Floaters [Member]

• Inverse Floaters. Inverse floating rate obligations are obligations that pay interest at rates that vary inversely with changes in market rates of interest. Because the interest rate paid to holders of such obligations is generally determined by subtracting a variable or floating rate from a predetermined amount, the interest rate paid to holders of such obligations will decrease as such variable or floating rate increases and increase as such variable or floating rate decreases.

(Morgan Stanley Mortgage Securities Trust - Classes A, L, I, C and R6) | (Mortgage Securities Trust) | U.S. Government Securities [Member]

• U.S. Government Securities. Different types of U.S. government securities are subject to different levels of credit risk, including the risk of default, depending on the nature of the particular government support for that security. For example, a U.S. government-sponsored entity, such as Federal National Mortgage Association or Federal Home Loan Mortgage Corporation, although chartered or sponsored by an Act of Congress, may issue securities that are neither insured nor guaranteed by the U.S. Treasury and, therefore, are not backed by the full faith and credit of the United States. With respect to U.S. government securities that are not backed by the full faith and credit of the United States, there is the risk that the U.S. government will not provide financial support to such U.S. government agencies, instrumentalities or sponsored enterprises if it is not obligated to do so by law. In addition, reduced participation in the repurchase agreement market by the Federal Reserve Bank of New York may affect the Fund’s investment strategies, operations and/or return potential. U.S. government securities are also subject to interest rate risks and can exhibit price fluctuations resulting from increases or decreases in interest rates.  

(Morgan Stanley Mortgage Securities Trust - Classes A, L, I, C and R6) | (Mortgage Securities Trust) | Asset-Backed Securities [Member]

• Asset-Backed Securities. Asset-backed securities are subject to credit (such as a borrower’s default on its mortgage obligation and the default or failure of a guarantee underlying the asset-backed security), interest rate and certain additional risks, including the risk that various federal and state consumer laws and other legal and economic factors may result in the collateral backing the securities being insufficient to support payment on the securities. Some asset-backed securities also entail prepayment risk and extension risk, which may vary depending on the type of asset. Due to these risks, asset-backed securities may become more volatile in certain interest rate environments.

(Morgan Stanley Mortgage Securities Trust - Classes A, L, I, C and R6) | (Mortgage Securities Trust) | Liquidity [Member]

• Liquidity. The Fund may make investments that are less liquid, illiquid or restricted or that may become illiquid or less liquid in response to overall economic conditions or adverse investor perceptions, and which may entail greater risk than investments in other types of securities. These investments may be more difficult to value or sell, particularly in times of market turmoil, and there may be little trading in the secondary market available for particular securities. Liquidity risk may be magnified in a market where credit spread and interest rate volatility is rising and where investor redemptions from fixed-income mutual funds may be higher than normal.  If the Fund is forced to sell an illiquid or restricted security to fund redemptions or for other cash needs, it may be forced to sell the security at a loss or for less than its fair value and may be unable to sell the security at all.

(Morgan Stanley Mortgage Securities Trust - Classes A, L, I, C and R6) | (Mortgage Securities Trust) | High Yield Securities (Junk Bonds) [Member]

• High Yield Securities (“Junk Bonds”). The Fund’s investments in high yield securities expose it to increased risks, including a substantial degree of credit and default risks. High yield securities may be issued by companies that are restructuring, are smaller and less  creditworthy or are more highly leveraged or indebted than other companies, and therefore they typically have more difficulty making scheduled payments of principal and interest than issuers of higher rated investments. High yield securities are subject to greater risk of loss (including substantial or total loss) of income and principal than higher rated securities and are considered speculative because of increased credit risk relative to higher rated fixed income investments. High yield securities are also subject to greater price volatility, including sudden and substantial decreases in price, and less liquidity than higher rated securities. High yield securities are particularly sensitive to adverse economic, market, industry or issuer-specific developments, which may result in an increased incidence of default. In the event of a default, the Fund may incur additional expenses to seek recovery or to negotiate new terms with a defaulting issuer.

(Morgan Stanley Mortgage Securities Trust - Classes A, L, I, C and R6) | (Mortgage Securities Trust) | Foreign Securities [Member]

• Foreign Securities. Investments in foreign markets entail special risks such as currency, political (including geopolitical), economic and market risks and heightened risks, that may result in losses to the Fund. There also may be greater market volatility, less

reliable financial information, less stringent investor protections and disclosure standards, higher transaction and custody costs, decreased market liquidity and less government and exchange regulation associated with investments in foreign markets. In addition, investments in certain foreign markets that have historically been considered stable may become more volatile and subject to increased risk due to developments and changing conditions in such markets. Moreover, the growing  interconnectivity of global economies and financial markets has increased the probability that adverse developments and conditions in one country or region will affect the stability of economies and financial markets in other countries or regions. Certain foreign markets may rely heavily on particular industries or foreign capital and are more vulnerable to diplomatic developments (including regional and global, military or other conflicts), the imposition of economic sanctions against a particular country or countries, organizations, companies, entities  and/or individuals, changes in international trading patterns, trade barriers (including tariffs) and other protectionist or retaliatory measures. Investments in foreign markets may also be adversely affected by governmental interventions or other actions such as the imposition of capital controls, nationalization of companies or industries, expropriation of assets or the imposition of punitive taxes. The governments of certain countries may prohibit or impose substantial restrictions on foreign investing in their capital markets or in certain sectors or industries. In addition, a foreign government may limit or cause delay in the convertibility or repatriation of its currency which would adversely affect the  U.S. dollar value and/or liquidity of investments denominated in that currency. Certain foreign investments may become less liquid and decline in value in response to market developments or adverse investor perceptions, or become illiquid after purchase by the Fund, particularly during periods of market, economic, political and social turmoil. When the Fund holds illiquid investments, its portfolio may be harder to value. In addition, the Fund’s investments in foreign issuers may be denominated in foreign currencies and therefore, to the extent unhedged, the value of those investments will fluctuate with U.S. dollar exchange rates. To the extent the Fund seeks to hedge its foreign currency exposure by the use of foreign currency forward exchange contracts, the precise matching of the foreign currency forward exchange contract amounts and the value of the securities involved will not generally be possible because the future value of such securities in foreign currencies will change as a consequence of market movements in the value of those securities between the date on which the contract is entered into and the date it matures. There is additional risk that such transactions may reduce or preclude the opportunity for gain if the value of the currency should move in the direction opposite to the position taken and that foreign currency forward exchange contracts create exposure to currencies in which the Fund’s securities are not denominated. The use of foreign currency forward exchange contracts involves the risks associated with derivatives and the risk of loss from the insolvency or bankruptcy of the counterparty to the contract or the failure of the counterparty to make payments or otherwise comply with the terms of the contract. Economic sanctions or other similar measures may be, and have been, imposed against certain countries, organizations, companies, entities and/or individuals. Economic sanctions and other similar measures could, among other things, effectively restrict or eliminate the Fund’s ability to purchase or sell securities (in the sanctioned country and other markets), negatively impact the value or liquidity of the Fund’s investments, significantly delay or prevent the settlement of the Fund’s securities transactions, force the Fund to sell or otherwise dispose of investments at inopportune times or prices, or impair the Fund’s ability to meet its investment objective or invest in accordance with its investment strategies.

(Morgan Stanley Mortgage Securities Trust - Classes A, L, I, C and R6) | (Mortgage Securities Trust) | Derivatives [Member]

• Derivatives. Derivatives and other similar instruments that create synthetic exposure often are subject to risks similar to those of the underlying asset or instrument, including market risk, and may be subject to additional risks, including imperfect correlation between the value of the derivative and the underlying asset, risks of default by the counterparty to certain transactions, magnification of losses incurred due to changes in the market value of the securities, instruments, indices or interest rates to which the derivative instrument relates, risks that the transactions may not be liquid, risks arising from margin and payment requirements, risks arising from mispricing or valuation complexity and operational and legal risks. Certain derivative transactions may give rise to a form of leverage. Leverage magnifies the potential for gain and the risk of loss. Investments in currency derivatives may substantially change the Fund’s exposure to currency exchange rates and could result in losses to the Fund if currencies do not perform as the Adviser expects.

(Morgan Stanley Mortgage Securities Trust - Classes A, L, I, C and R6) | (Mortgage Securities Trust) | Market and Geopolitical Risk [Member]

• Market and Geopolitical Risk. The value of your investment in the Fund is based on the values of the Fund’s investments, which change due to economic and other events that affect the U.S. and global markets generally, as well as those that affect or are perceived or expected to affect particular regions, countries, industries, companies, issuers, sectors, asset classes or governments. These types of events may be sudden and unexpected, and could adversely affect the value (or income generated by) and liquidity of the Fund’s investments, which may in turn impact the Fund’s ability to sell securities and/or its ability to meet redemptions. The risks associated with these developments may be magnified if certain social, political, economic and other conditions and events (such as war, natural disasters or events, epidemics and pandemics, terrorism, conflicts, social unrest, recessions, inflation, interest rate changes and supply chain disruptions) adversely interrupt or otherwise affect the global economy and financial markets. It is difficult to predict when events affecting the  U.S. or global financial markets or economies may occur, the effects that such events may have and the duration of those effects (which may last for extended periods). These types of events may negatively impact broad segments of businesses and populations and have a significant and rapid negative impact on the performance or value of the Fund’s investments,  adversely affect and increase the volatility of the Fund’s share price and exacerbate pre-existing risks to the Fund. The frequency and magnitude of resulting changes in the value of the Fund’s investments cannot be predicted.

(Morgan Stanley Mortgage Securities Trust - Classes A, L, I, C and R6) | (Mortgage Securities Trust) | Portfolio Turnover [Member]

• Portfolio Turnover. Consistent with its investment policies, the Fund will purchase and sell securities without regard to the effect on portfolio turnover. Higher portfolio turnover will cause the Fund to incur additional transaction costs.

(Morgan Stanley Mortgage Securities Trust - Classes A, L, I, C and R6) | (Mortgage Securities Trust) | Active Management Risk [Member]

• Active Management Risk. In pursuing the Fund’s investment objective, the Adviser has considerable leeway in deciding which investments to buy, hold or sell on a day-to-day basis, and which trading strategies to use. For example, the Adviser,  in its  discretion, may determine to use some permitted trading strategies while not using others. The success or failure of such decisions will affect the Fund’s performance.

(Morgan Stanley Mortgage Securities Trust - Classes A, L, I, C and R6) | (Mortgage Securities Trust) | Class A

Investments in Class A shares that are not subject to any sales charges at the time of purchase are subject to a contingent deferred sales charge (“CDSC”) of 0.75% that will be imposed if you sell your shares within 12 months after purchase, except for certain specific circumstances. See “Shareholder Information—Share Class Arrangements” for further information about the CDSC waiver categories.

(Morgan Stanley Mortgage Securities Trust - Classes A, L, I, C and R6) | (Mortgage Securities Trust) | Class C

The Class C CDSC is only applicable if you sell your shares within one year after the last day of the month of purchase. See “Shareholder Information—Share Class Arrangements” for a complete discussion of the CDSC.

[1] 1 During 2016, the Fund received proceeds related to certain non-recurring litigation settlements. Had these settlements not occurred, the 10 year and since inception (where applicable) returns before and after taxes for such periods would have been lower.