Immunoproteasome-Deficient Mice Mount Largely Normal CD8+ T Cell Responses to Lymphocytic Choriomeningitis Virus Infection and DNA Vaccination

Abstract

During viral infection, constitutive proteasomes are largely replaced by immunoproteasomes, which display distinct cleavage specificities, resulting in different populations of potential CD8+ T cell epitope peptides. Immunoproteasomes are believed to be important for the generation of many viral CD8+ T cell epitopes and have been implicated in shaping the immunodominance hierarchies of CD8+ T cell responses to influenza virus infection. However, it remains unclear whether these conclusions are generally applicable. In this study we investigated the CD8+ T cell responses to lymphocytic choriomeningitis virus infection and DNA immunization in wild-type mice and in mice lacking the immunoproteasome subunits LMP2 or LMP7. Although the total number of virus-specific cells was lower in LMP2 knockout mice, consistent with their having lower numbers of naive cells before infection, the kinetics of virus clearance were similar in all three mouse strains, and LMP-deficient mice mounted strong primary and secondary lymphocytic choriomeningitis virus-specific CD8+ T cell responses. Furthermore, the immunodominance hierarchy of the four investigated epitopes (nuclear protein 396 (NP396) > gp33 > gp276 > NP205) was well maintained. We observed a slight reduction in the NP205-specific response in LMP2-deficient mice, but this had no demonstrable biological consequence. DNA vaccination of LMP2- and LMP7-deficient mice induced CD8+ T cell responses that were slightly lower than, although not significantly different from, those induced in wild-type mice. Taken together, our results challenge the notion that immunoproteasomes are generally needed for effective antiviral CD8+ T cell responses and for the shaping of immunodominance hierarchies. We conclude that the immunoproteasome may affect T cell responses to only a limited number of viral epitopes, and we propose that its main biological function may lie elsewhere.

Proteasomes shape the repertoire of T cells participating in antigen-specific immune responses

“…Our data now provide direct evidence that this takes place. Furthermore, our data are also reconcilable with a report by Nussbaum et al (29), in which immunoproteasome-deficient mice basically show equal responses to lymphocytic choriomeningitis virus infection as infected WT mice. The authors conclude that immunoproteasomes may affect T cell responses to only a limited number of viral epitopes, and that the main biological function may lie elsewhere.…”

Proteasome immunosubunits protect against the development of CD8 T-cell-mediated autoimmune diseases

“…Also, in mouse models of experimental diabetes, except for those where foreign antigens are expressed from the rat insulin promoter, most described CD8 T cell targets are peptides that bind MHC-I molecules with low affinity (29) (30). The low affinity of the diabetogenic epitope IGRP 5′UTR [7][8][9][10][11][12][13][14] , targeted in our model (Figure 4), further supports this notion. Noteworthy, in a recent study (31), diabetes in RIP-B7 transgenic mice (32), induced by immunization with an ER-targeted preproinsulin cDNA construct that contained a low affinity H2-K b -presented diabetogenic CD8 T cell epitope, was prevented by co-immunization with a second cDNA construct encoding a peptide that bound this MHC-I molecule with high affinity.…”

“…To determine the relative affinity of IGRP 5′UTR [7][8][9][10][11][12][13][14] for MHC-I, we cultured the T cell line RMA-S, which lacks the transporter associated with antigen processing (TAP) and therefore expresses "empty" MHC-I molecules on its cell surface, overnight with synthetic IGRP 5′UTR [7][8][9][10][11][12][13][14] or with the control peptides OVA 257-264 or TRP2 180-188 (a self peptide derived from an melanoma-associated antigen). As shown in Figure 4d, IGRP 5′UTR 7-14 stabilized the H2-K b molecules on RMA-S, but to a lower extent than TRP2 180-188 or OVA [257][258][259][260][261][262][263][264] .…”

Endoplasmic reticulum aminopeptidase 1 (ERAP1) trims MHC class I-presented peptidesin vivoand plays an important role in immunodominance

“…The CTL response to LCMV, as with virtually all immune responses, is focused on a limited number of peptides, and, in mice with the same genetic background, the response to different peptides follows a strict and stable hierarchy, with (in C57BL͞6 mice) the peptide NP396 inducing the most CTL and GP118 the fewest (31). LCMV is a remarkably potent inducer of CTL, stimulating a CTL response and maintaining its immunodominance hierarchy in the absence of CD28 costimulation (34) and in the absence of immunoproteasomes (35) and showing only slight changes in immunodominance even in the absence of CD4 help (36). In ERAP1 Ϫ/Ϫ mice, however, the immunodominance hierarchy was disrupted: The response to several peptides was reduced by 1͞2 to 2͞3, whereas the response to other peptides was unchanged or even increased.…”