Citrullinated Epitopes Identified on Tumour MHC Class II by Peptide Elution Stimulate Both Regulatory and Th1 Responses and Require Careful Selection for Optimal Anti-Tumour Responses

Background

Somatic mutations or post-translational modifications of proteins result in changes that enable immune recognition. One such post-translational modification is citrullination, the conversion of arginine residues to citrulline. Citrullinated peptides are presented on MHC class II (MHCII) via autophagy which is upregulated by cellular stresses such as tumourigenesis.

Methods

Peptides were eluted from B16 melanoma expressing HLA-DP4 and analysed by mass spectrometry to profile the presented citrullinated repertoire. Initially, seven of the identified citrullinated peptides were used in combination to vaccinate HLA-DP4 transgenic mice. Immune responses were characterised from the combination and individual vaccines by ex vivo cytokine ELISpot assay and assessed for tumour therapy.

Results

The combination vaccine induced only weak anti-tumour therapy in the B16cDP4 melanoma model. Immune phenotyping revealed a dominant IFNγ response to citrullinated matrix metalloproteinase-21 peptide (citMMP21) and an IL-10 response to cytochrome p450 peptide (citCp450). Exclusion of the IL-10 inducing citCp450 peptide from the combined vaccine failed to recover a strong anti-tumour response. Single peptide immunisation confirmed the IFNγ response from citMMP21 and the IL-10 response from citCp450 but also showed that citrullinated Glutamate receptor ionotropic (citGRI) peptide stimulated a low avidity IFNγ response. Interestingly, both citMMP21 and citGRI peptides individually, stimulated strong anti-tumour responses that were significantly better than the combined vaccine. In line with the citGRI T cell avidity, it required high dose immunisation to induce an anti-tumour response. This suggests that as the peptides within the combined vaccine had similar binding affinities to MHC-II the combination vaccine may have resulted in lower presentation of each epitope and weak anti-tumour immunity.

Conclusion

We demonstrate that tumours present citrullinated peptides that can stimulate Th1 and regulatory responses and that competition likely exists between similar affinity peptides. Characterisation of responses from epitopes identified by peptide elution are necessary to optimise selection for tumour therapy.

Expression of the mouse MHC class Ib H2-T11 gene product, a paralog of H2-T23 (Qa-1) with shared peptide-binding specificity

The mouse MHC class Ib gene H2-T11 is 95% identical at the DNA level to H2-T23, which encodes Qa-1, one of the most studied MHC class Ib molecules. H2-T11 mRNA was observed to be expressed widely in tissues of C57BL/6 mice, with the highest levels in thymus. To circumvent the availability of a specific mAb, cells were transduced with cDNA encoding T11 with a substituted α3 domain. Hybrid T11D3 protein was expressed at high levels similar to control T23D3 molecules on the surface of both TAP(+) and TAP(-) cells. Soluble T11D3 was generated by folding in vitro with Qa-1 determinant modifier, the dominant peptide presented by Qa-1. The circular dichroism spectrum of this protein was similar to that of other MHC class I molecules, and it was observed to bind labeled Qa-1 determinant modifier peptide with rapid kinetics. By contrast to the Qa-1 control, T11 tetramers did not react with cells expressing CD94/NKG2A, supporting the conclusion that T11 cannot replace Qa-1 as a ligand for NK cell inhibitory receptors. T11 also failed to substitute for Qa-1 in the presentation of insulin to a Qa-1-restricted T cell hybridoma. Despite divergent function, T11 was observed to share peptide-loading specificity with Qa-1. Direct analysis by tandem mass spectrometry of peptides eluted from T11D3 and T23D3 isolated from Hela cells demonstrated a diversity of peptides with a clear motif that was shared between the two molecules. Thus, T11 is a paralog of T23 encoding an MHC class Ib molecule that shares peptide-binding specificity with Qa-1 but differs in function.

Differences in MHC-class I presented minor histocompatibility antigens extracted from normal and graft-versus-host disease (GVHD) mice

Graft-versus-host disease (GVHD) may develop after allogeneic bone marrow transplantation (BMT) between donors and recipients incompatible for minor histocompatibility antigens (mHAg). Here, we examined the possible relationship between tissue-specific distribution of dominant mHAg peptides and specific organ destruction caused by GVHD. In the B6 anti-Balb/b (H-2b) strain combination, a GVHD developed against Balb/b mHAgs. Despite the high number of incompatible mHAgs between these two strains, both cytotoxic T lymphocyte (CTL) response and GVHD could be attributed to a limited number of dominant mHAgs. We studied CTL-defined expression of dominant mHAgs in normal tissues and their GVHD-related modifications. mHAg peptides were prepared by acid elution and reversed-phase high pressure liquid chromatography fractionation from the spleen, liver, gut and skin as GVHD target tissues and from the heart and kidney as control tissues. Peptidic fractions extracted from normal and GVHD tissues were incubated with RMA-S targets and analysed using bulk B6 anti-Balb/b CTL. In each tissue several fractions were recognized with a given pattern of mHAg expression. GVHD induced qualitative and quantitative changes in antigenic peptide expression. Modifications in mHAg presentation during GVHD concerned preferentially GVHD target organs as opposed to non-GVHD target organs. In addition, when immunizing tissues were derived from GVHD mice instead of normal mice, the profile of CTL recognition was different. In conclusion, these data indicate that broad differences could exist in peptide presentation between various normal and GVHD-target organs.