Virionic HLA-C molecules associate to Env and increase the infectivity of both R5 and X4 viruses. Genetic polymorphisms associated to variations in HLA-C expression levels may therefore influence the individual viral set point not only by means of a regulation of the virus-specific immune response but also via a direct effect on the virus replicative capacity. These findings have implications for the understanding of the HIV-1 entry mechanism and of the role of Env conformational modifications induced by virion-associated host proteins.

The hypothesis that open conformers of HLA-C on target cells might directly exert an effect on their infectability by human immunodeficiency virus (HIV) has been suggested previously. This was tested by exploiting the peculiar specificity of monoclonal antibody (mAb) L31 for HLA-C open conformers to show that normal levels of Env-driven fusion were restored in HLA-C transfectants of a major histocompatibility complex-deleted (fusion-incompetent) cell line. The physiological relevance of this finding is now confirmed in this report, where small interfering RNA (siRNA) technology was used to silence HLA-C expression in peripheral blood lymphocytes (PBLs) from 11 healthy donors. Infectability by HIV (strains IIIB and Bal and primary isolates) was significantly reduced (P50.016) in silenced cells compared with cells that maintained HLA-C expression in 10 of the 11 PBL donors. Normal infectability was resumed, together with HLA-C expression, when the effect of siRNA interference waned after several days in culture. Additional confirmation of the HLA-C effect was obtained in several assays employing HLA-C-positive and -negative cell lines, a number of HIV strains and also pseudoviruses. In particular, viruses pseudotyped with env genes from HIV strains AC10 and QH0692.42 were assayed on siRNA-silenced lymphocytes from three healthy donors: the differences in infection with pseudoviruses were even higher than those observed in infections with normal viruses. INTRODUCTIONThree recent whole-genome association studies have demonstrated and confirmed that minor allele variants of singlenucleotide polymorphisms in the major histocompatibility complex (MHC; in particular HLA-C) gene regions are associated with a lower viral load at the set point and additionally with delayed human immunodeficiency virus type 1 (HIV-1) disease progression (Fellay et al., 2007;Limou et al., 2009;van Manen et al., 2009). These findings, which echo a previous report on the association between HLA-C alleles and rapid progression to AIDS (Carrington et al., 1999), have prompted wide speculations on the role of HLA (and HLA-C in particular) in HIV biology and immunology. HLA-C differs from HLA-A and -B because HIV-1 selectively downregulates HLA-A and -B, but does not significantly affect HLA-C (or HLA-E). Therefore, infected cells display reduced detection by both cytotoxic lymphocytes and natural killer (NK) cells (Cohen et al., 1999). The hypothesis that HLA-C might exert an effect directly on HIV infectivity was originally suggested by De Santis et al. (1996), exploiting the peculiar specificity of monoclonal antibody (mAb) L31 to show that normal levels of Env-driven fusion were restored in HLA-C transfectants (Grassi et al., 1991;Setini et al., 1996) of an MHC-deleted (fusion-incompetent) cell line. The physiological relevance of this finding is confirmed in this report, where small interfering RNA (siRNA) technology was used to silence HLA-C expression in peripheral blood lymphocytes (PBLs), the natural target of HIV-1. Two different kinds o...