Studies on HIV-2 antibody mediated neutralisation, coreceptor usage and in vivo tropism

Abstract

Human immunodeficiency virus type 2 (HIV-2) is the second virus that causes AIDS in humans. It has a genetic identity with HIV-1 of 40-60% and similar genetic organisation and biological properties, but the two viruses are distinguished certain features. HIV-2's geographical spread is mainly restricted to West Africa, the clinical latency period is significantly longer than for HIV-1, it has a reduced heterosexual and vertical transmission rate and lower levels plasma virus are found in infected individuals. The reasons for these differences are largely unknown. The aim of this thesis was to characterise different biological properties of HIV-2.

Since induction of broadly neutralising antibodies is a desirable feature of a future HIV-2 vaccine, knowledge of neutralising epitopes in the HIV-2 envelope proteins is important. By peptide immunisation of guinea pigs, the central and C-terminal part of the V3 region of HIV2 gp125 was confirmed to be an important target for neutralising antibodies. However, subtle changes in the sequence and length of peptides resulted in major differences in the ability to elicit HIV-2 neutralising antibodies. The conserved F-315, H-316, W-329 and C-330 amino acid residues were suggested to participate in a conformational neutralising epitope.

Production of recombinant human antibody Fab fragments by combinatorial library/phage display was shown to be a suitable method to obtain anti-HIV-2 antibodies with neutralising capacity. Six Fabs that neutralised the homologous strain SBL6669 were obtained, of which one also neutralised a heterologous virus isolate.

The tropism of HIV is largely determined by the coreceptor usage of the virus. Primary HIV-2 isolates were shown to frequently use CCR5, but were often promiscuous in coreceptor usage. Broadening of coreceptor usage was not associated with disease progression. CXCR4 usage was observed for some isolates recovered from patients with advanced disease, and appeared to correlate with positively charged amino acid residues at positions 314 andlor 313 in the V3 loop. Low level BOB expression in MT-2 cells coupled with promiscuous coreceptor usage among HIV-2 isolates was suggested to account for difficulties in clearly distinguishing distinct phenotypic groups in MT-2 cells. CCR5 or CXCR4 were shown to be required for efficient infection of PBMC by primary HIV-2 isolates in vitro. However, inefficient CCR5 and CXCR4 independent infection of PBMC was observed for the majority of isolates tested.

Productive HIV-2 infection in the brain was shown to be restricted to macrophages or microglia. Thus, the broad coreceptor usage and relative CD4 independence of HIV-2 in vitro appears to have little influence on the in vivo tropism, at least in the brain compartment. Other factors are therefore suggested to account for the higher frequency of encephalopathy observed in HIV-2 than in HIV-1 infection.