52   Genetic, Virological and Immunological Characterization of a Cohort of Long-Term Monogamous HIV Discordant Couples.

R. Braganza*, S. Beddows, S. Frazeo, T. Dong, T. Rostron, S. Rowland-Jones, J. Weber, and S. Fidler.
Imperial Coll. Sch. of Med. at St. Mary's, London and John Radcliffe Hosp., Oxford, UK.

Background:There is increasing evidence of HIV-1-specific immune responses in individuals who despite frequent HIV-1 sexual exposure remain seronegative (ESN). Much of this work has focused on a cohort of seronegative prostitutes in Nairobi exposed to different sources of HIV. Here we define a cohort of monogamous discordant couples to assess the parameters that may contribute to non-transmission.

Methods:8 couples were selected who despite regular unprotected sex over at least a 3 year period have not transmitted HIV-1 to their partner. The cohort comprises 6 British homosexual couples and 2 heterosexual African couples. Blood and semen samples were taken every 3 months. Patients were screened for known polymorphisms in genes encoding chemokines and their receptors. Infectivity assays were performed on CD8- depleted PBMCs from ESN and seronegative donors using patient virus isolates. Mock infectivity assays were run in parallel to approximate levels of cellular proliferation (MTT assay), CD4/CCR5 and CD4/CXCR4 co- expression andb-chemokine levels. Co-receptor usage of the patient-derived viruses was determined. ELISPOT assays were performed to quantify IFNgrelease following incubation with HIV-1 peptides encompassing known CTL epitopes. Proviral DNA from sequential PBMCs of the positive partner were sequenced for synonymous (ds) and non-synonymous (dn) changes to these epitopes.

Results:Infectivity data suggests that ESN CD4⁺T-cells were not differentially resistant to infection when compared to donor CD4⁺T-cells, once host genetic polymorphisms were taken into account. Most patient isolates used CCR5 for entry, though one patient was infected with a CXCR4 monotropic isolate. ELISPOT responses were seen in all couples, including peptides that encompass gag, pol, env and nef. Some ESNs maintained this response, whilst others showed a loss of peptide recognition. In some cases an accumulation of ds and dn changes in and around the epitope was seen.

Conclusion:A detailed examination of a highly selected discordant couple cohort, with good follow-up, may help to delineate factors involved in host genetic, virological and immunological non-transmission.