HLA-C expression levels and binding stability to β2m modulate HIV-1 infectivity F. Parolini1, P. Biswas2, M. Serena3, F. Sironi2, S. Mutascio1, V. Muraro4,E. Guizzardi4, M. Malnati2, A. Beretta2, M.G. Romanelli1, D. Zipeto1
1Università degli Studi di Verona, Neurosciences, Biomedicine and Movement Sciences, Verona, Italy, 2IRCCS Ospedale San Raffaele, Milan, Italy, 3University of Oxford, Oxford, United Kingdom, 4Service of Transfusion Medicine, AOUI Verona, Verona, Italy
Background: HLA-C expression levels lead to different HIV-1 infection out- comes. A higher expression is associated with a better activation of cytotoxic T lymphocytes (CTLs) and thus a better HIV-1 infection control. Vice versa, a lower HLA-C expression leads to a rapid progression toward AIDS. Thus, HLA-C highly and lowly expressed alleles are defined as protective and non-protective, respec- tively. Furthermore, different HLA-C alleles have different binding stabilities to β2microglobulin (β2m). Interestingly, HLA-C protective alleles are also those that bind β2m more efficiently, while the non-protective variants present more free chains (not bound to β2m) on the cell surface. It is also known that virions lacking HLA-C have reduced infectivity and increased susceptibility to neutralizing anti- bodies.
Methods: The A3.01 cell line and its HIV-1-infected counterpart ACH-2 were used as an in vitro infection model. 293T β2m negative cells, generated using CRISPR/ Cas9 system, were utilized to produce HIV-1 pseudoviruses. PBMC from healthy blood donors, harboring both protective or non-protective alleles, were exploited to characterize the proportion between HLA-C associated to β2m and HLA-C pres- ents as free chains on the cell surface. In addition, PBMC from the same donors were tested for their ability to support HIV-1 infection in vitro.
Results: HLA-C free chains, specifically more represented on the surface of in- fected cells, are responsible for the increase of virions’ infectivity. We observed that HIV-1 Env-pseudotyped viruses produced in β2m negative cells, thus lacking HLA-C on their envelope, are less infectious than those produced in the presence of β2m. In PBMC we found that protective HLA-C variants are more stably bound to β2m than non-protective ones and that HIV-1, in vitro, infects more efficiently PBMC harboring non-protective, weakly bound to β2m, HLA-C alleles.Conclusions: We propose that the outcome of HIV-1 infection might be driven both by the HLA-C surface expression levels and by the HLA-C/β2m binding stability. According to this model, the expression of non-protective HLA-C alleles, which bind weakly β2m, leads to a reduction of immunocompetent complexes expressed on the cell surface and to an increase of HLA-C free chains that raises viral infectiv- ity, both leading to a rapid progression toward AIDS.
