Abstract 608: Low avidity neoepitope-specific CD8+ tumor infiltrating lymphocytes are less exhausted and more effective than their high avidity counterparts in a BALB/c murine fibrosarcoma

The frequency of CD8+ tumor infiltrating lymphocytes (TILs) has been positively associated with overall survival in several cancers. The interaction of T cell receptors (TCRs) on T cells with Major Histocompatibility Complex (MHC)-peptide complexes on cancer cells elicits the cytotoxic activity of CD8+ T cells. Here we set out to understand how the strength of this interaction, or T cell avidity, shapes the phenotype of CD8+ TILs and how that dictates anti-tumor immunity. We developed a novel tetramer decay assay to isolate T cells based on their TCR avidities. We then used this method to isolate from tumors of Meth A tumor-bearing mice (28 days after tumor challenge), Low (K off < 15 min), Medium (75 min > K off > 45 min), and High (K off > 105 min) avidity CD8+ TILs responding to the tumor rejection neoepitope, PDPRMUT of this fibrosarcoma (Ebrahimi-Nik et al. JCI Insight 2019). Unbiased clustering of single cell RNA sequencing, TCR sequencing, and CITE-seq of these PDPRMUT -specific CD8+ TILs revealed five clusters: Stem-Like (LY108+Tcf7+), Effector-Exhausted (Eff-Exh PD-1Low,CX3CR1+) Terminally-Exhausted-1 (Term-Exh1 PD-1+TIM3High), Terminally-Exhausted-2 (Term-Exh2 PD-1HighToxHigh), and NK-Like (PD-1Low, KlrHigh). The low avidity CD8+ TILs have more than twice the proportion of Eff-Exh (60.0%) when compared to high avidity CD8+ TILs (28.39%). The high avidity CD8+ TILs have more Stem-Like (12.37% vs 6.31%), Term-Exh1 (27.38% vs 21.88%), and Term-Exh2 (24.03% vs 5.33%) cells than low avidity CD8+ TILs. Furthermore, trajectory inference of these clusters, by scVelo, predicts that Stem-Like cells differentiate into Eff-Exh cells that can differentiate into either Term-Exh1 or Term-Exh2. Our data demonstrates that low avidity Eff-Exh cells preferentially differentiate into Term-Exh1, while high avidity Eff-Exh cells differentiate into both Term-Exh1 and Term-Exh2. To validate these results, we isolated CD8+ TILs responding to PDPRMUT with low or high avidity from tumors of 21 Meth A bearing mice 28 days after tumor challenge and analyzed their exhaustion phenotype by flow cytometry. There were significantly more TIM3+PD-1+ cells (Paired t-test; P < 0.0001) and TOX+PD-1+ cells (Paired t-test; P < 0.05) in the high avidity CD8+ TILs than the low avidity CD8+ TILs. We conclude that CD8+ TILs with high avidity are more exhausted and less effector like than low avidity CD8+ TILs. This finding has major implications for T cell-based cancer immunotherapy.

Citation Format: Summit Singhaviranon, Joseph P. Dempsey, Adam T. Hagymasi, Pramod K. Srivastava. Low avidity neoepitope-specific CD8+ tumor infiltrating lymphocytes are less exhausted and more effective than their high avidity counterparts in a BALB/c murine fibrosarcoma [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2023; Part 1 (Regular and Invited Abstracts); 2023 Apr 14-19; Orlando, FL. Philadelphia (PA): AACR; Cancer Res 2023;83(7_Suppl):Abstract nr 608.

GeNeo: A Bioinformatics Toolbox for Genomics-Guided Neoepitope Prediction

High-throughput DNA and RNA sequencing are revolutionizing precision oncology, enabling personalized therapies such as cancer vaccines designed to target tumor-specific neoepitopes generated by somatic mutations expressed in cancer cells. Identification of these neoepitopes from next-generation sequencing data of clinical samples remains challenging and requires the use of complex bioinformatics pipelines. In this paper, we present GeNeo, a bioinformatics toolbox for genomics-guided neoepitope prediction. GeNeo includes a comprehensive set of tools for somatic variant calling and filtering, variant validation, and neoepitope prediction and filtering. For ease of use, GeNeo tools can be accessed via web-based interfaces deployed on a Galaxy portal publicly accessible at https://neo.engr.uconn.edu/. A virtual machine image for running GeNeo locally is also available to academic users upon request.

Heat-Shock Proteins

Heat-shock proteins (HSPs), or stress proteins, are abundant and highly conserved, present in all organisms and in all cells. Selected HSPs, also known as chaperones, play crucial roles in folding and unfolding of proteins, assembly of multiprotein complexes, transport and sorting of proteins into correct subcellular compartments, cell-cycle control and signaling, and protection of cells against stress and apoptosis. More recently, HSPs have been shown to be key players in immune responses: during antigen presentation as well as cross-priming, they chaperone and transfer antigenic peptides to class I and class II molecules of the major histocompatibility complexes. In addition, extracellular HSPs can stimulate and cause maturation of professional antigen-presenting cells of the immune system, such as macrophages and dendritic cells. They also chaperone several toll-like receptors, which play a central role in innate immune responses. HSPs constitute a large family of proteins that are often classified based on their molecular weight as Hsp10, Hsp40, Hsp60, Hsp70, Hsp90, etc. This unit contains a table that lists common HSPs and summarizes their characteristics including (a) name, (b) subcellular localization, (c) known function, (d) chromosome assignment, (e) brief comments, and (f) references. © 2022 Wiley Periodicals LLC.

Reversion analysis reveals the in vivo immunogenicity of a poorly MHC I-binding cancer neoepitope

High-affinity MHC I-peptide interactions are considered essential for immunogenicity. However, some neo-epitopes with low affinity for MHC I have been reported to elicit CD8 T cell dependent tumor rejection in immunization-challenge studies. Here we show in a mouse model that a neo-epitope that poorly binds to MHC I is able to enhance the immunogenicity of a tumor in the absence of immunization. Fibrosarcoma cells with a naturally occurring mutation are edited to their wild type counterpart; the mutation is then re-introduced in order to obtain a cell line that is genetically identical to the wild type except for the neo-epitope-encoding mutation. Upon transplantation into syngeneic mice, all three cell lines form tumors that are infiltrated with activated T cells. However, lymphocytes from the two tumors that harbor the mutation show significantly stronger transcriptional signatures of cytotoxicity and TCR engagement, and induce greater breadth of TCR reactivity than those of the wild type tumors. Structural modeling of the neo-epitope peptide/MHC I pairs suggests increased hydrophobicity of the neo-epitope surface, consistent with higher TCR reactivity. These results confirm the in vivo immunogenicity of low affinity or ‘non-binding’ epitopes that do not follow the canonical concept of MHC I-peptide recognition.

The immunogenicity of peptides is believed to be determined by their high-affinity binding to MHC I. Here authors show that low-affinity MHC I-peptide interactions are also able to trigger robust T cell response and anti-tumour immunity in vivo.

An unbiased approach to defining bona fide cancer neoepitopes that elicit immune-mediated cancer rejection

Identification of neoepitopes that are effective in cancer therapy is a major challenge in creating cancer vaccines. Here, using an entirely unbiased approach, we queried all possible neoepitopes in a mouse cancer model and asked which of those are effective in mediating tumor rejection and, independently, in eliciting a measurable CD8 response. This analysis uncovered a large trove of effective anticancer neoepitopes that have strikingly different properties from conventional epitopes and suggested an algorithm to predict them. It also revealed that our current methods of prediction discard the overwhelming majority of true anticancer neoepitopes. These results from a single mouse model were validated in another antigenically distinct mouse cancer model and are consistent with data reported in human studies. Structural modeling showed how the MHC I-presented neoepitopes had an altered conformation, higher stability, or increased exposure to T cell receptors as compared with the unmutated counterparts. T cells elicited by the active neoepitopes identified here demonstrated a stem-like early dysfunctional phenotype associated with effective responses against viruses and tumors of transgenic mice. These abundant anticancer neoepitopes, which have not been tested in human studies thus far, can be exploited for generation of personalized human cancer vaccines.

Unbiased analysis of all possible neoepitopes of MC38-FABF tumor reveals a new universe of cancer neoepitopes with unexpected properties

Cancer neoepitopes are the only truly tumor-specific antigens and therefore, the most suitable as cancer vaccines. Current methods to predict neoepitopes, based on studies of viral epitopes, emphasize high affinity MHC-peptide interactions. Increasing evidence in human and murine models indicates that the present neoepitope prediction methods are mostly inaccurate in predicting true MHC I-restricted cancer neoepitopes. Here, in a completely unbiased approach, all possible neoepitopes in a mouse tumor model were tested for their ability to mediate tumor rejection and also CD8+ T cell responses. These studies show that the true tumor rejecting neoepitopes have different properties from those of viral epitopes. Further, CD8+ T cell responses elicited by these neoepitopes possess a more plastic chromatin phenotype with stem-like properties that is known to be associated with anti-viral and anti-tumor immune responses. Such cancer neoepitopes can be exploited for generation of personalized cancer vaccines.

Unbiased testing of all single nucleotide variants specific to a tumor for anti-tumor CD8+ T cell immune response

Neoepitopes presented in the context of MHC render tumors immunogenic. However, not all potential neoepitopes elicit immunity, and the attributes of immunogenic neoepitopes are yet to be fully defined. Based on the premise that regressing tumors must elicit T cells that recognize the true anti-tumor neoepitopes, we have developed and used here an ELISPOT based high-throughput assay that allows testing of the immunogenicity of a large number of candidate neoepitopes. Naïve C57BL/6 mice were challenged with the colorectal tumor line MC38 and treated, starting day 5 post-tumor challenge, with a checkpoint-blocking antibody, leading to the beginning of tumor regression. MC38-specific CD8+ T cell response was observed in tumor-bearing hosts with regressing tumors. Exome and transcriptome sequencing of MC38 and accompanying bioinformatic analyses have revealed >800 single nucleotide variations (SNVs) in MC38. To identify the SNVs contributing to the immunogenicity of MC38, reactivity of the anti-MC38 CD8+ T cells isolated from mice with regressing tumors, towards individual peptides corresponding to each identified SNV, was tested. Indeed, CD8+ T cells against a number of mutant peptides were detected. These immunogenic mutant peptides are potential MHC I-restricted neoepitopes of MC38 and their role in tumor rejection is being tested.

Unbiased testing of several hundred tumor-specific single nucleotide variants of a tumor for protective immunogenicity and CD8+ response reveals surprises

Somatic mutations in cancer cells can give rise to new MHC epitopes referred to as neoepitopes. Neoepitopes have been clearly demonstrated to elicit an antigen specific T cell response, which are capable of mediating targeted killing of cancer cells. Despite recent advancements, exactly what criteria make a good neoepitope for personalized therapy is currently unknown. Here, using a syngeneic tumor of C57BL/6 mice, named FABF, we present results of an exhaustive and exhausting study where we have tested hundreds of long peptides each containing a single nucleotide variant (SNV) (with the mutation in the center of the peptide) for their ability to elicit tumor rejection and independently, CD8+ T cell response. We observe that (i) about 2% of all SNVs lead to generation of peptides that can mediate tumor rejection to any significant degree; (ii) each peptide alone elicits a modest protection, and a combination elicits stronger protective immunity, (iii) all the neoepitopes that elicit tumor rejection have poor binding affinity for MHC I, and have positive values for Differential Agretopic Index (1). (iv) Even though the protective responses are CD8-mediated, there is no correlation between a neoepitope’s ability to elicit a CD8 response and tumor rejection. Some of these observations are inconsistent with some aspects of the current consensus about the defining characteristics of good neoepitopes.

An Immunotherapeutic CD137 Agonist Releases Eomesodermin from ThPOK Repression in CD4 T Cells

Agonists to the TNF/TNFR costimulatory receptors CD134 (OX40) and CD137 (4-1BB) elicit antitumor immunity. Dual costimulation with anti-CD134 plus anti-CD137 is particularly potent because it programs cytotoxic potential in CD8⁺ and CD4⁺ T cells. Cytotoxicity in dual-costimulated CD4 T cells depends on the T-box transcription factor eomesodermin (Eomes), which we report is induced via a mechanism that does not rely on IL-2, in contrast to CD8⁺ CTL, but rather depends on the CD8 T cell lineage commitment transcription factor Runx3, which supports Eomes expression in mature CD8⁺ CTLs. Further, Eomes and Runx3 were indispensable for dual-costimulated CD4 T cells to mediate antitumor activity in an aggressive melanoma model. Runx3 is also known to be expressed in standard CD4 Th1 cells where it fosters IFN-γ expression; however, the CD4 T cell lineage commitment factor ThPOK represses transcription of Eomes and other CD8 lineage genes, such as Cd8a Hence, CD4 T cells can differentiate into Eomes⁺ cytotoxic CD4⁺CD8⁺ double-positive T cells by terminating ThPOK expression. In contrast, dual-costimulated CD4 T cells express Eomes, despite the continued expression of ThPOK and the absence of CD8α, indicating that Eomes is selectively released from ThPOK repression. Finally, although Eomes was induced by CD137 agonist, but not CD134 agonist, administered individually, CD137 agonist failed to induce CD134^-/- CD4 T cells to express Eomes or Runx3, indicating that both costimulatory pathways are required for cytotoxic Th1 programming, even when only CD137 is intentionally engaged with a therapeutic agonist.

CD134 plus CD137 dual costimulation induces Eomesodermin in CD4 T cells to program cytotoxic Th1 differentiation

Cytotoxic CD4 Th1 cells are emerging as a therapeutically useful T cell lineage that can effectively target tumors, but until now the pathways that govern their differentiation have been poorly understood. We demonstrate that CD134 (OX40) costimulation programs naive self- and virus-reactive CD4 T cells to undergo in vivo differentiation into cytotoxic Th1 effectors. CD137 (4-1BB) costimulation maximized clonal expansion, and IL-2 was necessary for cytotoxic Th1 differentiation. Importantly, the T-box transcription factor Eomesodermin was critical for inducing the cytotoxic marker granzyme B. CD134 plus CD137 dual costimulation also imprinted a cytotoxic phenotype on bystanding CD4 T cells. Thus, to our knowledge, the current study identifies for the first time a specific costimulatory pathway and an intracellular mechanism relying on Eomesodermin that induces both Ag-specific and bystander cytotoxic CD4 Th1 cells. This mechanism might be therapeutically useful because CD134 plus CD137 dual costimulation induced CD4 T cell-dependent tumoricidal function in a mouse melanoma model.

The E3 ubiquitin ligase Cbl-b regulates expansion but not functional activity of self-reactive CD4 T cells

Cbl-b is an E3 ubiquitin ligase that limits Ag responsiveness in T cells by targeting TCR-inducible signaling molecules. Cbl-b deficiency thus renders T cells hyperresponsive to antigenic stimulation and predisposes individuals toward developing autoimmunity. In part because Cbl-b(-/-) T cells do not require CD28 costimulation to become activated, and insufficient costimulation is a critical parameter that confers anergy induction over effector differentiation, it has been hypothesized that Cbl-b(-/-) T cells are resistant to anergy. This possibility has been supported in models in which anergy is normally induced in vitro, or in vivo following exposure to soluble Ag boluses. In the current study, we characterized the response of Cbl-b(-/-) CD4 T cells in an in vivo system in which anergy is normally induced by a constitutively expressed peripheral self-Ag. Cbl-b expression increased in self-Ag-induced anergic wild-type CD4 T cells, and Cbl-b(-/-) CD4 T cells underwent more robust proliferation and expansion upon initially encountering cognate self-Ag compared with wild-type counterparts. Nevertheless, both wild-type and Cbl-b(-/-) CD4 T cells ultimately developed the same impaired ability to respond to antigenic restimulation. The more extensive expansion that occurred during the initial induction of anergy did, however, allow the anergic CD4 T cells to expand to greater numbers when they were functionally resuscitated following replacement of the initial source of tolerizing self-Ag with a viral form of the same Ag.

A novel sphingomyelinase-like enzyme in Ixodes scapularis tick saliva drives host CD4 T cells to express IL-4

Tick feeding modulates host immune responses. Tick-induced skewing of host CD4(+) T cells towards a Th2 cytokine profile facilitates transmission of tick-borne pathogens that would otherwise be neutralized by Th1 cytokines. Tick-derived factors that drive this Th2 response have not previously been characterized. In the current study, we examined an I. scapularis cDNA library prepared at 18-24 h of feeding and identified and expressed a tick gene with homology to Loxosceles spider venom proteins with sphingomyelinase activity. This I. scapularis sphingomyelinase-like (IsSMase) protein is a Mg(2+)-dependent, neutral (pH 7.4) form of sphingomyelinase. Significantly, in an in vivo TCR transgenic adoptive transfer assay IsSMase programmed host CD4(+) T cells to express the hallmark Th2 effector cytokine IL-4. IsSMase appears to directly programme host CD4 T cell IL-4 expression (as opposed to its metabolic by-products) because induced IL-4 expression was not altered when enzymatic activity was neutralized. TCR transgenic CD4 T cell proliferation (CFSE-dilution) was also significantly increased by IsSMase. Furthermore, a Th2 response is superimposed onto a virally primed Th1 response by IsSMase. Thus, IsSMase is the first identified tick molecule capable of programming host CD4(+) T cells to express IL-4.

Self-antigen prevents CD8 T cell effector differentiation by CD134 and CD137 dual costimulation

We compared how CD4 vs CD8 cells attain the capacity to express the effector cytokine IFN-gamma under both immunogenic and tolerogenic conditions. Although the Ifng gene locus was epigenetically repressed in naive Ag-inexperienced CD4 cells, it had already undergone partial remodeling toward a transcriptionally competent configuration in naive CD8 cells. After TCR stimulation, CD8 cells fully remodeled the Ifng locus and gained the capacity to express high levels of IFN-gamma more rapidly than CD4 cells. Enforced dual costimulation through OX40 and 4-1BB redirected CD8 cells encountering soluble exogenous peptide to expand and differentiate into IFN-gamma and TNF-alpha double-producing effectors rather than becoming tolerant. Despite this and the stronger tendency of CD8 compared with CD4 cells to differentiate into IFN-gamma-expressing effectors, when parenchymal self-Ag was the source of tolerizing Ag, enforced dual costimulation selectively boosted expansion but did not push effector differentiation in CD8 cells while both expansion and effector differentiation were dramatically boosted in CD4 cells. Notably, enforced dual costimulation was able to push effector differentiation in CD8 cells encountering cognate parenchymal self-Ag when CD4 cells were simultaneously engaged. Thus, the ability of enforced OX40 plus 4-1BB dual costimulation to redirect CD8 cells to undergo effector differentiation was unexpectedly influenced by the source of tolerizing Ag and help was selectively required to facilitate CD8 cell effector differentiation when the tolerizing Ag derived from self.

Histone acetylation at the Ifng promoter in tolerized CD4 cells is associated with increased IFN-gamma expression during subsequent immunization to the same antigen

When naive CD4(+) Th cells encounter cognate pathogen-derived Ags they expand and develop the capacity to express the appropriate effector cytokines for neutralizing the pathogen. Central to this differentiation process are epigenetic modifications within the effector cytokine genes that allow accessibility to the transcriptional machinery. In contrast, when mature self-reactive CD4 cells encounter their cognate epitopes in the periphery they generally undergo a process of tolerization in which they become hyporesponsive/anergic to antigenic stimulation. In the current study, we used a TCR transgenic adoptive transfer system to demonstrate that in a dose-dependent manner parenchymal self-Ag programs cognate naive CD4 cells to acetylate histones bound to the promoter region of the Ifng gene (which encodes the signature Th1 effector cytokine) during peripheral tolerization. Although the Ifng gene gains transcriptional competence, these tolerized CD4 cells fail to express substantial amounts of IFN-gamma in response to antigenic stimulation apparently because a blockage in TCR-mediated signaling also develops. Nevertheless, responsiveness to antigenic stimulation is partially restored when self-Ag-tolerized CD4 cells are retransferred into mice infected with a virus expressing the same Ag. Additionally, there is preferential boosting in the ability of these CD4 cells to express IFN-gamma relative to other cytokines with expression that also becomes impaired. Taken together, these results suggest that epigenetic modification of the Ifng locus during peripheral CD4 cell tolerization might allow for preferential expression of IFN-gamma during recovery from tolerance.

Steady state dendritic cells present parenchymal self-antigen and contribute to, but are not essential for, tolerization of naive and Th1 effector CD4 cells

Bone marrow-derived APC are critical for both priming effector/memory T cell responses to pathogens and inducing peripheral tolerance in self-reactive T cells. In particular, dendritic cells (DC) can acquire peripheral self-Ags under steady state conditions and are thought to present them to cognate T cells in a default tolerogenic manner, whereas exposure to pathogen-associated inflammatory mediators during the acquisition of pathogen-derived Ags appears to reprogram DCs to prime effector and memory T cell function. Recent studies have confirmed the critical role of DCs in priming CD8 cell effector responses to certain pathogens, although the necessity of steady state DCs in programming T cell tolerance to peripheral self-Ags has not been directly tested. In the current study, the role of steady state DCs in programming self-reactive CD4 cell peripheral tolerance was assessed by combining the CD11c-diphtheria toxin receptor transgenic system, in which DC can be depleted via treatment with diphtheria toxin, with a TCR-transgenic adoptive transfer system in which either naive or Th1 effector CD4 cells are induced to undergo tolerization after exposure to cognate parenchymally derived self-Ag. Although steady state DCs present parenchymal self-Ag and contribute to the tolerization of cognate naive and Th1 effector CD4 cells, they are not essential, indicating the involvement of a non-DC tolerogenic APC population(s). Tolerogenic APCs, however, do not require the cooperation of CD4(+)CD25(+) regulatory T cells. Similarly, DC were required for maximal priming of naive CD4 cells to vaccinia viral-Ag, but priming could still occur in the absence of DC.

Dendritic cells program non-immunogenic prostate-specific T cell responses beginning at early stages of prostate tumorigenesis

BACKGROUND

Prostate cancer promotes the development of T cell tolerance towards prostatic antigens, potentially limiting the efficacy of prostate cancer vaccines targeting these antigens. Here, we sought to determine the stage of disease progression when T cell tolerance develops, as well as the role of steady state dendritic cells (DC) and CD4(+)CD25(+) T regulatory cells (Tregs) in programming tolerance.

METHODS

The response of naïve HA-specific CD4(+) T cells were analyzed following adoptive transfer into Pro-HA x TRAMP transgenic mice harboring variably-staged HA-expressing prostate tumors on two genetic backgrounds that display different patterns and kinetics of tumorigenesis. The role of DC and Tregs in programming HA-specific CD4 cell responses were assessed via depletion.

RESULTS

HA-specific CD4 cells underwent non-immunogenic responses at all stages of tumorigenesis in both genetic backgrounds. These responses were completely dependent on DC, but not appreciably influenced by Tregs.

CONCLUSIONS

These results suggest that tolerogenicity is an early and general property of prostate tumors.

T-bet down-modulation in tolerized Th1 effector CD4 cells confers a TCR-distal signaling defect that selectively impairs IFN-gamma expression

When Th1 effector CD4 cells encounter tolerizing Ag in vivo, their capacity to express the effector cytokines IFN-gamma and TNF-alpha is lost more rapidly than noneffector functions such as IL-2 production and proliferation. To localize the relevant intracellular signaling defects, cytokine expression was compared following restimulation with Ag vs agents that bypass TCR-proximal signaling. IFN-gamma and TNF-alpha expression were both partially rescued when TCR-proximal signaling was bypassed, indicating that both TCR-proximal and -distal signaling defects impair the expression of these two effector cytokines. In contrast, bypassing TCR-proximal signaling fully rescued IL-2 expression. T-bet, a transcription and chromatin remodeling factor that is required to direct the differentiation of naive CD4 cells into IFN-gamma-expressing Th1 effectors, was partially down-modulated in tolerized Th1 effectors. Enforcing T-bet expression during tolerization selectively rescued the ability to express IFN-gamma, but not TNF-alpha. Conversely, expression of a dominant-negative T-bet in Th1 effectors selectively impaired the ability to express IFN-gamma, but not TNF-alpha. Analysis of histone acetylation at the IFN-gamma promoter further suggested that down-modulation of T-bet expression during Th1 effector CD4 cell tolerization does not impair IFN-gamma expression potential through alterations in chromatin structure.

Glycoprotein 96 can chaperone both MHC class I- and class II-restricted epitopes for in vivo presentation, but selectively primes CD8+ T cell effector function

The ability of mature T lymphocytes to develop effector capacity after encounter with cognate Ag is generally dependent upon inflammatory signals associated with infection that induce dendritic cell activation/maturation. These inflammatory signals can derive directly from pathogens or can be expressed by host cells in response to infection. Heat shock proteins (HSPs) are a class of host-derived inflammatory mediators that perform the dual function of both chaperoning MHC class I-restricted epitopes into the cross-presentation pathway of DCs and inducing the activation/maturation of these DCs to allow priming of cognate CD8(+) T cell effector responses. Although the ability of HSPs to elicit effector CD8 cell responses has been well established, their potential to prime CD4 cell effector responses has been relatively unexplored. In the current study we compared the ability of the endoplasmic reticulum-resident HSP gp96 to prime CD4 vs CD8 cells using TCR transgenic adoptive transfer systems and soluble gp96-peptide complexes. As expected, gp96 facilitated the cross-presentation of a class I-restricted peptide and priming of effector function in cognate CD8 cells. Interestingly, gp96 also facilitated the in vivo presentation of a class II-restricted peptide; however, the resulting CD4 cell response did not involve the development of effector function. Taken together, these data suggest that gp96 is an inflammatory mediator that selectively primes CD8 cell effector function.