Comprehensive Analysis of CD8 T Cell Immune Response Specific for Two Novel HLA-A*0201 Restriced CMV pp65 Peptides.

In immune compromised subjects such as patients undergoing bone marrow, organ transplantation or immunosuppressive therapies, Cytomegalovirus (CMV) infection is associated with significant morbidity until the individual’s immune system is completely reconstituted. One method of preventing CMV infection during immune suppression in transplant patients is represented by adoptive administration of CMV peptide-specific cytotoxic T lymphocytes (CTLs) from HLA-matched donors. Despite the strong immunogenicity demonstrated by specific HLA class I peptides, the peptide’s responsiveness varies among individuals. Therefore, it is important to identify additional epitopes for each HLA determinant of interest. In this study we report a comprehensive analysis of CD8 T cell-specific activity against two novel CMV pp65 HLA-A*0201 associated peptides in CMV-experienced HLA-A*0201 subjects. PBMCs from CMV seropositive HLA-A*0201 restricted healthy subjects were peptide-stimulated ex vivo and IFN-γ gene expression was analyzed by qrt-PCR. An IFN-γ ELISPOT assay was carried out on peptide-specific elicited CD8 T lymphocytes to confirm the qrt-PCR results. Tetrameric HLA/epitope complexes (tHLA) were used to track the levels of peptide-specific CD8 T cells responsiveness to cognate epitopes. In addition, in vitro peptide-specific expanded populations of CTLs were used in ⁵¹Cr release assay. Based on qrt-PCR results, four out of eight HLA-A*0201 peptides identified by computer algorithms were selected. Two are preaviously published peptides: pp65_495–503 (NLVPMVATV) and pp65_347–355 (ALFFFDIDL), while two are novel: pp65_340–348 (RQYDPVAAL) and pp65_310–318 (LMNGQQIFL). In spite the four peptides induced comparable mRNA IFN-γ transcript production, peptides pp65_495–503, pp65_340–348 and pp65_310–318 induced a consistent and sustained IFN-γ protein release and specific killing, while the pp65_347–355 failed to induce IFN-γ protein secretion and killing activity (if we exclude a positive IFN-γ protein release after 2-week in vitro induction in one of the donors tested). Comparative assays carried on the functional activity of the three peptides pp65_495–503, pp65_340–348 and pp65_310–318 revealed no intrinsic differences in term of IFN-γ protein release and cytotoxic activity save for the CTL affinity to the HLA-A*0201/epitope complexes (pp65_495–503 ~2.6% in nearly 100% of donors vs pp65_340–348 ~0.67% or pp65_310–318 ~0.77% in nearly one third of the donors after 2-week in vitro induction). The tHLA binding results could be possibly ascribed to differencies in the peptide avidity and stability for the HLA class I. Taken together, these results lead to the conclusion that different peptides can induce a variable levels of immune responses ranging between mere cytokine gene expression and effective cytotoxic activity. In addition, the two novel peptides selected here broaden the panel of potential reagents useful for adoptive immune therapy. Thus, in anticipation of a specific epitope-targeted immune intervention, the three HLA-A*0201 peptides described could be used in combination for adoptive transfer of epitope-specific T cells or epitope-specific vaccination.