Peptide-MHC-I from Endogenous Antigen Outnumber Those from Exogenous Antigen, Irrespective of APC Phenotype or Activation.

Janet J. Sei,Clifford J. Bellone,R. Mark Buller,Lauren W. Kaminsky,Christopher C. Norbury,Scott Haskett,Eugene Lin,Mary E. Truckenmiller,Chris A. Benedict

doi:10.1371/journal.ppat.1004941

Janet J. Sei, Clifford J. Bellone + Show 7 more

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https://doi.org/10.1371/journal.ppat.1004941

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Abstract

Naïve anti-viral CD8+ T cells (TCD8+) are activated by the presence of peptide-MHC Class I complexes (pMHC-I) on the surface of professional antigen presenting cells (pAPC). Increasing the number of pMHC-I in vivo can increase the number of responding TCD8+. Antigen can be presented directly or indirectly (cross presentation) from virus-infected and uninfected cells, respectively. Here we determined the relative importance of these two antigen presenting pathways in mousepox, a natural disease of the mouse caused by the poxvirus, ectromelia (ECTV). We demonstrated that ECTV infected several pAPC types (macrophages, B cells, and dendritic cells (DC), including DC subsets), which directly presented pMHC-I to naïve TCD8+ with similar efficiencies in vitro. We also provided evidence that these same cell-types presented antigen in vivo, as they form contacts with antigen-specific TCD8+. Importantly, the number of pMHC-I on infected pAPC (direct presentation) vastly outnumbered those on uninfected cells (cross presentation), where presentation only occurred in a specialized subset of DC. In addition, prior maturation of DC failed to enhance antigen presentation, but markedly inhibited ECTV infection of DC. These results suggest that direct antigen presentation is the dominant pathway in mice during mousepox. In a broader context, these findings indicate that if a virus infects a pAPC then the presentation by that cell is likely to dominate over cross presentation as the most effective mode of generating large quantities of pMHC-I is on the surface of pAPC that endogenously express antigens. Recent trends in vaccine design have focused upon the introduction of exogenous antigens into the MHC Class I processing pathway (cross presentation) in specific pAPC populations. However, use of a pantropic viral vector that targets pAPC to express antigen endogenously likely represents a more effective vaccine strategy than the targeting of exogenous antigen to a limiting pAPC subpopulation.

Highlights

In the fight against virus invasion, TCD8+ play an essential role by killing virus-infected cells
The extent to which presentation by infected or uninfected cells contribute to the induction of a protective CD8+ T cell response has not been studied extensively during a natural infection in a mouse model
The importance of this work lies in the fact that, if infected cells present way more antigen than uninfected cells, successful vaccine design should utilize this observation to make a vaccine where infected cells expressing virus proteins are the prevalent mode of induction of CD8+ T cells

Summary

Introduction

In the fight against virus invasion, TCD8+ play an essential role by killing virus-infected cells. We have demonstrated that presentation of endogenously synthesized antigen results in much higher pMHC-I levels than acquisition of exogenous antigen and that, on a per cell basis, each infected pAPC population produces equivalent pMHC-I levels, irrespective of activation or maturation status. These data have important ramifications for rational vaccine design in that they indicate that a vaccine in which endogenous synthesis of the targeted antigen occurs within multiple pAPC populations is the most effective way to generate the greatest number of effective pMHC-I complexes which, in turn, results in an optimal antigen specific TCD8+ response

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