# EDGAR Filing Document

**Accession Number:** 0000939934
**File Stem:** 0001104659-25-096498
**Filing Date:** 2025-10
**Character Count:** 26600
**Document Hash:** e08e73d3d55f96713279fc58c927092b
**Contains OCR:** False
**Source Format:** 

## Filing Content

## Filing Summary
**0001104659-25-096498.hdr.sgml**: 20251003

**ACCESSION NUMBER**: 0001104659-25-096498

**CONFORMED SUBMISSION TYPE**: 497K

**PUBLIC DOCUMENT COUNT**: 3

**FILED AS OF DATE**: 20251003

**DATE AS OF CHANGE**: 20251003

**EFFECTIVENESS DATE**: 20251003

**FILER**: 

**COMPANY DATA:**
- **COMPANY CONFORMED NAME:** SEI INSTITUTIONAL INVESTMENTS TRUST
- **CENTRAL INDEX KEY:** 0000939934

**ORGANIZATION NAME:**
- **EIN:** 000000000
- **STATE OF INCORPORATION:** MA
- **FISCAL YEAR END:** 0531

**FILING VALUES:**
- **FORM TYPE:** 497K
- **SEC ACT:** 1933 Act
- **SEC FILE NUMBER:** 033-58041
- **FILM NUMBER:** 251372740

**BUSINESS ADDRESS:**
- **STREET 1:** 1 FREEDOM VALLEY DRIVE
- **CITY:** OAK
- **STATE:** PA
- **ZIP:** 19456
- **BUSINESS PHONE:** 610 676-3097

**MAIL ADDRESS:**
- **STREET 1:** 1 FREEDOM VALLEY DRIVE
- **CITY:** OAK
- **STATE:** PA
- **ZIP:** 19456

## Series and Classes Contracts Data

### SIIT LONG DURATION FUND (Series ID: S000006762)

| Class ID   | Class Name                        | Ticker Symbol   |
|:---|:---|:---|
| C000018351 | SIIT LONG DURATION FUND - CLASS A | LDRAX           |

![](j252262341_ba001.jpg)

September 30, 2025

SUMMARY PROSPECTUS

SIIT Long Duration Fund (LDRAX)

Class A

Before you invest, you may want to review the Fund's Prospectus, which contains information about the Fund and its risks. You can find the Fund's Prospectus and other information about the Fund, including the Fund's Statement of Additional Information and annual and semi-annual reports, online at seic.com/fundprospectuses. You can also get this information at no cost by dialing 1-800-DIAL-SEI. The Fund's Prospectus and Statement of Additional Information, dated September 30, 2025, as may be supplemented from time to time, are incorporated by reference into this Summary Prospectus and may be obtained, free of charge, at the website or phone number noted above.

seic.com

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SEI / SUMMARY PROSPECTUS

Investment Goal

Return characteristics similar to those of high quality bonds.

Fees and Expenses

This table describes the fees and expenses that you may pay if you buy, hold, and sell Fund shares. You may pay other fees, such as brokerage commissions and other fees to financial intermediaries, which are not reflected in this table and examples below.

ANNUAL FUND OPERATING EXPENSES

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| | |
|:---|:---|
| (expenses that you pay each year as a percentage of the value of your investment) | Class A Shares |
| Management Fees | 0.30% |
| Distribution (12b-1) Fees |  |
| Other Expenses | 0.09% |
| Total Annual Fund Operating Expenses | 0.39% |

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EXAMPLE

This Example 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 for the time periods indicated and then redeem or hold all of your shares at the end of those periods. The Example also assumes that your investment has a 5% return each year and that the Fund's operating expenses remain the same. Although your actual costs may be higher or lower, based on these assumptions your costs would be:

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| | | | | |
|:---|:---|:---|:---|:---|
| | 1 Year | 3 Years | 5 Years | 10 Years |
| Long Duration Fund — Class A Shares | $40 | $125 | $219 | $493 |

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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 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 176% of the average value of its portfolio.

Principal Investment Strategies

Under normal circumstances, the Long Duration Fund will invest at least 80% of its net assets (plus the amount of any borrowings for investment purposes) in investment grade US dollar-denominated fixed income instruments. The Fund will primarily invest in long duration government and corporate fixed income securities and may also invest in synthetic instruments or derivatives having economic characteristics similar to fixed income securities. The Fund will invest in a broad array of fixed income instruments including: (i) US and foreign corporate obligations; (ii) securities issued or guaranteed by the U.S. Government and its agencies and instrumentalities; (iii) fixed income securities issued by sovereigns or agencies in both developed and emerging foreign markets; (iv) obligations of supranational entities; and (v) debt obligations issued by state, provincial, county or city governments or other municipalities, as well as those of public utilities, universities and other quasi-governmental bodies.

The Fund will primarily invest in the instruments described above. It may also invest in futures contracts, forward contracts, and swaps, including interest rate swaps, single security swaps, swaps on an index of securities or credit default swaps. The Fund will primarily use such derivatives for hedging purposes to attempt to manage the Fund's exposure to changes in interest rate duration and related investment risks resulting from the interaction of interest rate changes over time and the current value of fixed income securities. The Fund will typically use options and swaps in an attempt to either mitigate the Fund's overall level of risk or to gain exposure to a particular fixed income security or segment of the fixed income market. Interest rate swaps, credit default swaps and total return swaps may be used to manage various portfolio exposures including but not limited to interest rate risk and credit risk. The Fund may use credit default swaps to take an active long or short position with respect to a security or basket of securities. The Fund may buy credit default swaps in an attempt to manage credit risk where the Fund has credit exposure to an issuer and the Fund may sell credit default swaps to more efficiently gain credit exposure to such security or basket of securities. The Fund may also, to a lesser extent, invest in shares of exchange-traded funds (ETFs) or mutual funds to obtain exposure to certain fixed income markets.

While the Fund may invest in securities with any maturity or duration, the Fund under normal circumstances will seek to maintain an effective average duration of greater than ten years. The Fund's effective average duration was approximately 12.68 years as of July 31, 2025. Duration is a measure of the expected life of a fixed income security that is used to determine the sensitivity of a

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SEI / SUMMARY PROSPECTUS

security's price to changes in interest rates. For example, if a fixed income security has a ten-year duration, it will decrease in value by 10% if interest rates rise 1% and increase in value by 10% if interest rates fall 1%. Fixed income instruments with higher duration typically have higher risk and higher volatility.

The Fund uses a multi-manager approach under the general supervision of SEI Investments Management Corporation (SIMC or the Adviser), allocating its assets among one or more sub-advisers (each, a Sub-Adviser and collectively, the Sub-Advisers) using different investment strategies designed to provide current income consistent with the preservation of capital. Due to its investment strategy, the Fund may buy and sell securities frequently.

Principal Risks

*Market Risk* — The prices of the Fund's fixed income securities respond to economic developments, particularly interest rate changes, as well as to perceptions about the creditworthiness of individual issuers, including governments and their agencies. Generally, the Fund's fixed income securities will decrease in value if interest rates rise and vice versa. In a low interest rate environment, risks associated with rising rates are heightened. Declines in dealer market-making capacity as a result of structural or regulatory changes could decrease liquidity and/or increase volatility in the fixed income markets. Markets for fixed income securities may decline significantly in response to adverse issuer, political, regulatory, market, economic or other developments that may cause broad changes in market value, public perceptions concerning these developments, and adverse investor sentiment or publicity. Similarly, environmental and public health risks, such as natural disasters, epidemics, pandemics or widespread fear that such events may occur, may impact markets adversely and cause market volatility in both the short- and long-term. In response to these events, the Fund's value may fluctuate and/or the Fund may experience increased redemptions from shareholders, which may impact the Fund's liquidity or force the Fund to sell securities into a declining or illiquid market.

*Duration Risk* — The longer-term securities in which the Fund may invest are more volatile. A portfolio with a longer average portfolio duration is more sensitive to changes in interest rates than a portfolio with a shorter average portfolio duration.

*Investment Style Risk* — The risk that longer-term U.S. fixed income securities may underperform other segments of the fixed income markets or the fixed income markets as a whole.

*Interest Rate Risk* — The risk that a change in interest rates will cause a fall in the value of fixed income securities, including U.S. Government securities, in which a Fund invests. Generally, the value of the Fund's fixed income securities will vary inversely with the direction of prevailing interest rates. Changing interest rates may have unpredictable effects on the markets and may affect the value and liquidity of instruments held by the Fund.

*U.S. Government Securities Risk* — Although U.S. Government securities are considered to be among the safest investments, they are still subject to the credit risk of the U.S. Government and are not guaranteed against price movements due to changing interest rates. Obligations issued by some U.S. Government agencies are backed by the U.S. Treasury, while others are backed solely by the ability of the agency to borrow from the U.S. Treasury or by the agency's own resources. No assurance can be given that the U.S. Government will provide financial support to its agencies and instrumentalities if it is not obligated by law to do so.

*Corporate Fixed Income Securities Risk* — Corporate fixed income securities respond to economic developments, especially changes in interest rates, as well as perceptions of the creditworthiness and business prospects of individual issuers.

*Foreign Investment/Emerging Markets Risk* — The risk that non-U.S. securities may be subject to additional risks due to, among other things, political, social and economic developments abroad, currency movements, and different legal, regulatory, tax, accounting and audit environments. These additional risks may be heightened with respect to emerging market countries because political turmoil and rapid changes in economic conditions are more likely to occur in these countries. Investments in emerging markets are subject to the added risk that information in emerging market investments may be unreliable or outdated due to differences in regulatory, accounting or auditing and financial record keeping standards, or because less information about emerging market investments is publicly available. In addition, the rights and remedies associated with emerging market investments may be different than investments in developed markets. A lack of reliable information, rights and remedies increase the risks of investing in emerging markets in comparison to more developed markets. In addition, periodic U.S. Government restrictions on investments in issuers from certain foreign countries may require the Fund to sell such investments at inopportune times, which could result in losses to the Fund.

*Foreign Sovereign Debt Securities Risk* — The risks that (i) the governmental entity that controls the repayment of sovereign debt may not be willing or able to repay the principal and/or interest when it becomes due, because of factors such as debt service burden, political constraints, cash flow problems and other national economic factors; (ii) governments may default on their debt securities, which may require holders of such securities to participate in debt rescheduling or additional lending to defaulting governments; and (iii) there is no bankruptcy proceeding by which defaulted sovereign debt may be collected in whole or in part.

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SEI / SUMMARY PROSPECTUS

*Municipal Securities Risk* — Municipal securities, like other fixed income securities, rise and fall in value in response to economic and market factors, primarily changes in interest rates, and actual or perceived credit quality. Rising interest rates will generally cause municipal securities to decline in value. Longer-term securities respond more sharply to interest rate changes than do shorter-term securities. A municipal security will also lose value if, due to rating downgrades or other factors, there are concerns about the issuer's current or future ability to make principal or interest payments. State and local governments rely on taxes and, to some extent, revenues from private projects financed by municipal securities, to pay interest and principal on municipal debt. Poor statewide or local economic results or changing political sentiments may reduce tax revenues and increase the expenses of municipal issuers, making it more difficult for them to repay principal and to make interest payments on securities owned by the Fund. Actual or perceived erosion of the creditworthiness of municipal issuers may reduce the value of the Fund's holdings. As a result, the Fund will be more susceptible to factors that adversely affect issuers of municipal obligations than a mutual fund that does not have as great a concentration in municipal obligations.

*Derivatives Risk* — The Fund's use of futures, forwards and swaps is subject to market risk, leverage risk, correlation risk and liquidity risk. Leverage risk and liquidity risk are described below and market risk is described above. Many over-the-counter (OTC) derivative instruments will not have liquidity beyond the counterparty to the instrument. Correlation risk is the risk that changes in the value of the derivative may not correlate perfectly with the underlying asset, rate or index. The Fund's use of forward contracts and swaps is also subject to credit risk and valuation risk. Credit risk is described below. Valuation risk is the risk that the derivative may be difficult to value and/or valued incorrectly. Some derivatives have the potential for unlimited loss, regardless of the size of the Fund's initial investment. The other parties to certain derivative contracts present the same types of credit risk as issuers of fixed income securities. The Fund's use of derivatives may also increase the amount of taxes payable by shareholders. Both U.S. and non-U.S. regulators have adopted and implemented regulations governing derivatives markets, the ultimate impact of which remains unclear.

*Exchange-Traded Funds Risk* — The risks of owning shares of an ETF generally reflect the risks of owning the underlying securities the ETF is designed to track, although lack of liquidity in an ETF could result in its value being more volatile than the underlying portfolio securities. When the Fund invests in an ETF, in addition to directly bearing the expenses associated with its own operations, it will bear a pro rata portion of the ETF's expenses.

*Currency Risk* — As a result of the Fund's investments in securities or other investments denominated in, and/or receiving revenues in, foreign currencies the Fund will be subject to currency risk. Currency risk is the risk that foreign currencies will decline in value relative to the U.S. dollar or, in the case of hedging positions, that the U.S. dollar will decline in value relative to the currency hedged. In either event, the dollar value of an investment in the Fund would be adversely affected. Due to the Fund's active positions in currencies, it will be subject to the risk that currency exchange rates may fluctuate in response to, among other things, changes in interest rates, intervention (or failure to intervene) by U.S. or foreign governments, central banks or supranational entities, or by the imposition of currency controls or other political developments in the United States or abroad.

*Liquidity Risk* — The risk that certain securities may be difficult or impossible to sell at the time and the price that the Fund would like. The Fund may have to lower the price of the security, sell other securities instead or forego an investment opportunity, any of which could have a negative effect on Fund management or performance.

*Credit Risk* — The risk that the issuer of a security or the counterparty to a contract will default or otherwise become unable to honor a financial obligation.

*Extension Risk* — The risk that rising interest rates may extend the duration of a fixed income security, typically reducing the security's value.

*Prepayment Risk* — The risk that, in a declining interest rate environment, fixed income securities with stated interest rates may have the principal paid earlier than expected, requiring the Fund to invest the proceeds at generally lower interest rates.

*Leverage Risk* — The Fund's use of derivatives may result in the Fund's total investment exposure substantially exceeding the value of its portfolio securities and the Fund's investment returns depending substantially on the performance of securities that the Fund may not directly own. The use of leverage can amplify the effects of market volatility on the Fund's share price and may also cause the Fund to liquidate portfolio positions when it would not be advantageous to do so in order to satisfy its obligations. The Fund's use of leverage may result in a heightened risk of investment loss.

*Portfolio Turnover Risk* — Due to its investment strategy, the Fund may buy and sell securities frequently. This may result in higher transaction costs and taxes subject to ordinary income tax rates as opposed to more favorable capital gains rates, which may affect the Fund's performance.

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SEI / SUMMARY PROSPECTUS

*Investing in the Fund involves risk, and there is no guarantee that the Fund will achieve its investment goal. You could lose money on your investment in the Fund, just as you could with other investments. An investment in the Fund is not a deposit of a bank and is not insured or guaranteed by the Federal Deposit Insurance Corporation or any other government agency.*

Performance Information

The bar chart and the performance table below provide some indication of the risks of investing in the Fund by showing changes in the Fund's performance from year to year for the past ten calendar years and by showing how the Fund's average annual returns for 1, 5 and 10 years and since the Fund's inception compare with those of a broad measure of market performance. The performance information shown is based on full calendar years. The Fund's past performance (before and after taxes) is not necessarily an indication of how the Fund will perform in the future. For current performance information, please call 1-800-DIAL-SEI.

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| | |
|:---|:---|
| ![](j252262341_bd002.jpg)  | Best Quarter: 12.96% (12/31/23)<br>Worst Quarter: -12.61% (6/30/22)<br>The Fund's total return from January 1, 2025 to June 30, 2025 was 3.78%. |

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

This table compares the Fund's average annual total returns to those of the Bloomberg U.S. Aggregate Bond Index (a broad-based securities market index) as well as a blended benchmark that is composed of the Bloomberg U.S. Long Credit Index and the Bloomberg U.S. Long Government Index weighted 70%/30%. In prior years, the Fund also compared its performance to the Bloomberg U.S. Long Government/Credit Index.

After-tax returns are calculated using the historical highest individual federal marginal income tax rates and do not reflect the impact of state and local taxes. Your actual after-tax returns will depend on your tax situation and may differ from those shown. After-tax returns shown are not relevant to investors who hold their Fund shares through tax-deferred arrangements, such as 401(k) plans or individual retirement accounts. In some cases, the Fund's return after taxes may exceed the Fund's return before taxes due to an assumed tax benefit from any losses on a sale of Fund shares at the end of the measurement period.

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| | | | | |
|:---|:---|:---|:---|:---|
| Long Duration Fund | 1 Year | 5 Years | 10 Years | Since<br>Inception<br>(4/21/2004) |
| Return Before Taxes | -2.93% | -2.26% | 1.78% | 4.06% |
| Return After Taxes on Distributions | -4.82% | -4.45% | -0.66% | 1.59% |
| Return After Taxes on Distributions and Sale of Fund Shares | -1.72% | -2.34% | 0.51% | 2.23% |
| Bloomberg U.S. Aggregate Bond Index Return (reflects no deduction for fees, expenses or taxes) | 1.25% | -0.33% | 1.35% | 3.10% |
| The Fund's 70/30 Blended Benchmark Return (reflects no deduction for fees, expenses or taxes) | -3.33% | -2.81% | 1.36% | 4.56% |

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Management

Investment Adviser and Portfolio Managers. SEI Investments Management Corporation

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| | | |
|:---|:---|:---|
| Portfolio Manager | Experience with the Fund | Title with Adviser |
| Richard A. Bamford | Since 2014 | Portfolio Manager |
| Anthony Karaminas, CFA | Since 2021 | Portfolio Manager, Head of Sub-Advised Fixed Income |

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SEI / SUMMARY PROSPECTUS

Sub-Advisers and Portfolio Managers.

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| | | | |
|:---|:---|:---|:---|
| Sub-Adviser | Portfolio Manager | Experience <br>with the Fund | Title with Sub-Adviser |
| Income Research + Management | James Gubitosi, CFA<br>Michael Sheldon, CFA<br>Jake Remley, CFA | Since 2017<br>Since 2017<br>Since 2019 | Co-Chief Investment Officer, Chair of Investment <br>Committee<br>Co-Chief Investment Officer<br>Senior Portfolio Manager, Director of Investment <br>Strategy |
| Jennison Associates LLC | James Gaul, CFA<br>Miriam Zussman\*<br>Eric G. Staudt, CFA<br>Samuel B. Kaplan, CFA<br>Dmitri Rabin, CFA<br>David Morse, CFA<br>Natalia Glekel, CFA<br>Griffin Sullivan, CFA<br>Adriano Taylor-Escribano | Since 2016<br>Since 2012<br>Since 2011<br>Since 2016<br>Since 2019<br>Since 2020<br>Since 2022<br>Since 2024<br>Since 2025 | Head of Fixed Income, Managing Director and <br>Fixed Income Credit Portfolio Manager<br>Managing Director and Fixed Income Credit <br>Portfolio Manager<br>Managing Director and Fixed Income Credit <br>Portfolio Manager<br>Managing Director and Fixed Income Rates and <br>Securitized Portfolio Manager<br>Managing Director and Fixed Income Rates and <br>Securitized Portfolio Manager<br>Managing Director and Fixed Income Credit <br>Portfolio Manager<br>Managing Director and Fixed Income Credit <br>Portfolio Manager<br>Managing Director and Fixed Income Credit <br>Portfolio Manager<br>Managing Director and Fixed Income Rates and <br>Securitized Portfolio Manager |
| Legal & General Investment <br>Management America, Inc. | Jason Shoup<br>Tim Bacik, CFA<br>Jordan Bond<br>Patrick Dan<br>Magdalena Szudy<br>Felipe Telles, CFA | Since 2023<br>Since 2011<br>Since 2017<br>Since 2017<br>Since 2023<br>Since 2024 | Chief Investment Officer, Co-Head of Global Fixed <br>Income<br>Head of Active Fixed Income<br>Senior Portfolio Manager<br>Head of Investment Grade Portfolio Management<br>Portfolio Manager<br>Senior Portfolio Manager |
| Metropolitan West Asset <br>Management, LLC | Bryan Whalen, CFA<br>Ruben Hovhannisyan, CFA<br>Jerry Cudzil | Since 2004<br>Since 2023<br>Since 2023 | Chief Investment Officer — Fixed Income, Group <br>Managing Director, Generalist Portfolio Manager<br>Group Managing Director, Generalist Portfolio <br>Manager<br>Group Managing Director, Generalist Portfolio <br>Manager |

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\* Ms. Zussman has announced her retirement from Jennison Associates LLC and will no longer serve as a portfolio manager effective on or about December 31, 2025.

Purchase and Sale of Fund Shares

The Fund's minimum investment requirements for Class A Shares are: (a) that you must be an Eligible Investor (*i.e.*, institutions or other SIMC advisory clients that have entered into an investment management agreement with SIMC or institutional investors, employee benefit plans and other similar entities purchasing through approved intermediaries); and (b) that your minimum initial investment must be $100,000, with minimum subsequent investments of $1,000, which may be waived at the discretion of SIMC. You may purchase and redeem shares of the Fund on any day that the New York Stock Exchange (NYSE) is open for business (a Business Day). You may sell your Fund shares by contacting your authorized financial institution or intermediary directly. Authorized financial institutions and intermediaries may redeem Fund shares on behalf of their clients by contacting the Fund's transfer agent (the Transfer Agent) or the Fund's authorized agent, using certain SEI Investments Company (SEI) or third party systems or by calling 1-800-858-7233, as applicable.

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SEI / SUMMARY PROSPECTUS

Tax Information

The distributions made by the Fund are generally taxable and will be taxed as qualified dividend income, ordinary income or capital gains. If you are investing through a tax-deferred arrangement, such as a 401(k) plan or individual retirement account, you will generally not be subject to federal taxation on Fund distributions until you begin receiving distributions from your tax-deferred arrangement. You should consult your tax advisor regarding the rules governing your tax-deferred arrangement.

Payments to Broker-Dealers and Other Financial Intermediaries

If you purchase Fund shares through a broker-dealer or other financial intermediary, such as a bank, the Fund and its related companies may pay the intermediary for the sale of Fund shares and related services. These payments may create a conflict of interest by influencing the broker-dealer or other intermediary and your salesperson to recommend the Fund over another investment. Ask your salesperson or visit your financial intermediary's website for more information.

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