P26.17
Background: Conventional HIV-1 proteins are thought to be derived from open reading frames (ORF). In addition to ORFs, HIV has the potential to transcribe and translate from five additional reading frames termed ARFs or alternate reading frames. Transcription and translation from ARFs has been shown to occur commonly during HIV, SIV, and other viral infections. As a result of ARF usage, antigenic peptides termed cryptic epitopes (CE) are generated, and becomes a potential source of HLA presented peptides. It is now well established that HIV often mutates to evade T cell recognition. However, many of these escape mutations are detrimental to viral fitness. We therefore hypothesize that CEs that induces viral escape at a population level must be important for controlling HIV.
Methods: In order to identify CEs that have escaped T cell recognition at a population level, viral sequences from a Zambian cohort consisting of 40 heterosexual serodiscordant couples were analyzed in addition to each person's HLA allelic information. This revealed multiple HLA-I restricted CEs within gag, nef, and pol regions of HIV-1. To validate our finding, 66 cryptic peptides were chosen and used to stimulate cryopreserved PBMC samples from HIV-1 chronically infected off-ART individuals.
Results: IFNγ ELISpot assay revealed HIV-1 CE specific responses at the peptide pool level in 9/32 (28%) individuals tested. However, such responses were not seen in the 16 HIV seronegative donors. We further mapped the positive responders and identified eleven responses at the single peptide level (median magnitude: 133 SFU/10^6 cells; range: 63 - 487 SFU/10^6 cells). Additionally, preliminary flow cytometric results revealed CE to also have elicited immune responses in CD4 T cells.
Conclusions: Despite their infrequent expression, our data suggests that CEs can enhance breadth of HIV specific T-cell responses. Additionally, since these epitopes were predicted based on HLA associated escape mutations, it further underscores their role in HIV control.
