Receptor use of primate lentiviruses

Abstract

Human and simian immunodeficiency viruses (HIV and SIV) enter target cells using CD4, the primary cell surface receptor and a secondary receptor, called coreceptor. Coreceptor use of HIV can be related to other biological features of the virus and to the severity of infection as well. Animal models are important in understanding transmission, pathogenesis and immune responses of HIV infection but are essential in studying the effect of candidate drugs or vaccines. The most widely used model for HIV infection in humans is SIV infection of macaques, therefore it is important to understand and compare the biological features of the two systems. The aim of this thesis was to study the role of coreceptors in HIV and SIV pathogenesis.

By the use of the GHOST(3) cell lines it was possible to set up a quantitative evaluation system. This allowed the detection of even slight changes in the coreceptor use of multicoreceptor tropic isolates obtained from individuals at different time points. Coreceptor use of HIV-1 isolates was studied in the context of mother-to-child transmission. 28 pregnant Cameroonian women and their newborn children were followed. Viruses from most of the patients (23 women) belonged to HIV-1 of genetic subtype A. Isolates predominantly used CCR5 (R5) for cell entry (26 out of 28 patients). No CXCR4 using virus could be isolated from any of the 23 patients harbouring subtype A viruses. Instead, CXCR6 (X6) use was evident in four cases; all of these isolates were R5X6 dual-tropic. Mother-to-child transmission occurred in four cases. Interestingly, prevalence of CXCR6 using viruses was higher among transmitting mothers (2/4) as compared to non-transmitting mothers (2/22). Since trophoblastic cells may express the CXCR6 receptor, we speculate that CXCR6 use by maternal viruses might be an advantage in crossing the placental barrier, thereby increasing the risk of mother to child transmission of HIV-1.

Coreceptor use of SIVsm inoculum viruses and sequential isolates from experimentally infected macaques with fast or slow disease progression and known neutralizing antibody pattern was also studied. Furthermore, envelopes (Envs) were cloned from isolates and tested for their ability to mediate fusion. This allowed us to study an expanded range of receptors and to test the capacity of receptors to mediate CD4-independent fusion. Envs were multi-coreceptor tropic and fused efficiently with a number of receptors. CCR5 use was efficient and stable in all macaques, whereas the use of CXCR6, BOB/gpr15, gpr1 and Apj decreased over time. Coreceptor usage pattern of virus isolates and Env clones was similar in all monkeys. Therefore no changes were found that could be related to SIV pathogenesis.

CD4-independent fusion with CCR5 was significant and revealed an interesting pattern. Envs from inoculum viruses showed distinct dependencies on CD4 for fusion via CCR5. Envs from early re-isolates maintained or increased CD4-independent use of CCR5 and displayed high variability in this regard. In contrast, Envs from late re-isolates tended to show less variation and increased CD4 dependence if derived from animals that were able to mount neutralizing antibody response to their virus. This is consistent with findings that show neutralization sensitivity of CD4-independent viruses and suggests that CD4-dependent (and neutralization resistant) viruses were selected in the macaques over time. Infection with a strictly CD4-dependent virus resulted in evolution of CD4-independent Envs over time.

The high level of virus replication during acute infection and the high rate of mutation during replication may result in the appearance of a highly variable virus population. Our results highlight the role of neutralizing antibodies in the control of SIV infection. Receptor tropism, availability of target cells and immune activation may shape the course of viral evolution and SIV pathogenesis.