# Mechanisms of cross-presenting antigens in phagosomes on MHC I molecules to stimulate CD8 T lymphocyte responses

> **NIH NIH R01** · UNIV OF MASSACHUSETTS MED SCH WORCESTER · 2022 · $418,750

## Abstract

Abstract
In most cells the MHC I antigen presentation pathway exclusively displays peptides that are derived from a cell's
own proteins. In contrast, dendritic cells (DCs) and macrophages (MØs) are capable of acquiring and then
displaying peptides from external antigens through a process called cross presentation (XPT). XPT is the key
mechanism that allows the immune system to recognize and then mobilize a CD8 T cell response to cancers,
many viral infections and intracellular pathogens. Consequently, this pathway is important for immune
surveillance and is an attractive target to enable vaccines to elicit CD8 T cell immunity, which is something
current subunit vaccines fail to do. The overall goal of this grant is to elucidate key mechanisms that allow DCs
to carry out this critical function. In the major XPT pathway, exogenous proteins are first internalized into
phagosomes and then transferred into the cytosol, where they are cleaved into oligopeptides by proteasomes.
Recent data suggests that the proteasome-generated peptides for XPT are subsequently imported back into
phagosomes for binding to MHC I molecules (referred to here as “phagosomal XPT”). However, why such
peptides would not simply be delivered to MHC I molecules in the ER, as most cytosolic generated peptides are,
is a mystery and one that our first aim seeks to solve. Our underlying hypothesis, supported by preliminary data,
is that a subset of proteasomes physically associates with the cytosolic face of phagosomes and does so by via
their PA28 capping complex binding to the cytosolic domains of the peptide-loading complex (TAP+Tapasin).
This arrangement thereby links local peptide generation (by the phagosome-bound proteasomes) to local peptide
transport (by phagosomal TAP). The importance of these hypotheses is that they have the potential to fill in key
missing links in the phagosomal XPT pathway and to identify an important function for PA28 complexes, which
up until now have thought to be relatively unimportant. Our second aim seeks to elucidate mechanisms that allow
MHC I molecules to bind peptides in the “unfriendly” environment of the phagosome and to address the question
of why XPT of antigens is surprisingly much more efficient when the exogenous antigen is cell-associated, as
compared to the same antigen in any other form. This aim will test the hypothesis that ß2 microblobulin (ß2M)
from ingested exogenous cells promotes the formation of peptide-MHC I complexes in phagosomes. In this
mechanism, free ß2M (from ingested cells) + free MHC I heavy chains delivered to and/or generated in
phagosomes (from complexes denatured in the vacuole) + peptides (from PA28-proteasome-products that are
imported into phagosomes by TAP) associate to form intraphagosomal MHC I-peptide complexes. The
importance of this hypothesis is that it would provide insight into a biologically important process. Moreover, this
mechanism may be able to be manipulated to enhance XPT for vaccines/immunot...

## Key facts

- **NIH application ID:** 10392945
- **Project number:** 5R01AI145932-04
- **Recipient organization:** UNIV OF MASSACHUSETTS MED SCH WORCESTER
- **Principal Investigator:** KENNETH L ROCK
- **Activity code:** R01 (R01, R21, SBIR, etc.)
- **Funding institute:** NIH
- **Fiscal year:** 2022
- **Award amount:** $418,750
- **Award type:** 5
- **Project period:** 2019-05-24 → 2024-04-30

## Primary source

NIH RePORTER: https://reporter.nih.gov/project-details/10392945

## Citation

> US National Institutes of Health, RePORTER application 10392945, Mechanisms of cross-presenting antigens in phagosomes on MHC I molecules to stimulate CD8 T lymphocyte responses (5R01AI145932-04). Retrieved via AI Analytics 2026-06-11 from https://api.ai-analytics.org/grant/nih/10392945. Licensed CC0.

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