Militancia performativa: el abuso identitario P B Preciado

At the time of selection, it was known that there must be coreceptors for HIV in addition to CD4, but what these were was still an enigma. Thus, it was not possible to choose g p l20 on the basis of which chemokine receptor they bound. However, it had been shown that HIV strains fell into two broadly representative groups, those that could infect T-cell lines (T-tropic) and those that were able to infect primary macrophages (M-tropic), and that this tropism was primarily defined by g p l20 (Hwang et a i, 1991, Shioda et ai,

1991). Furthermore T-tropism and M-tropism broadly correlated with the ability (SI) or inability (NSI), respectively, to form syncytia (Fenyo et a i, 1989) as outlined in section 1.5.4. Thus g p l20 from two different strains of HIV were chosen, one being T-tropic and SI and the other being M-tropic and NSI, with the hope that they would be broadly representative.

3.1.1.1- SF2 and SF162

SF2, formerly known as ARV-2, and SF162 are two strains of HIV that were isolated from separate patients in San Francisco. SF2 was isolated from the peripheral blood of an individual with candidiasis (Levy et aL, 1984), and SF162 from the cerebrospinal fluid of an individual with toxoplasmosis (Cheng-Mayer et a i, 1989). For both, molecular clones have been constructed and sequenced. Hence their genetic make-up has remained stable over many generations (Cheng-Mayer et aL, 1989, Sanchez Pescador et a i, 1985). These strains have the advantage that they have been much studied and thus a good deal is known about their biology.

(Shioda era/., 1991). However, SF162 does not replicate in T-cell lines nor pro-monocytic cell lines such as U937 (which, although macrophage-related, resemble T-cell lines in their pattern of permissiveness for HIV strains), but does replicate well in primary monocyte- derived macrophages, MDM, and microglial cells (Cheng-Mayer et al., 1989, Collman et a l, 1989, Shioda et a l, 1991). Conversely SF2 does not replicate at all in MDM, but replicates well in T-cell lines and U937 cells (Cheng-Mayer et al, 1990, Shioda et al,

1991).

As expected SF2 is SI, with CD4+ T cells and T cell lines, whereas SF162 is classified as NSI (Broder & Berger, 1995, Cheng-Mayer etal, 1990). Further to this, if cells expressing recombinant envs were made, those expressing recombinant SF162 env were able to fuse with MDM and not T-cell lines, and for those expressing SF2 env the opposite selectivity pattern was observed (Broder & Berger, 1995). Hence SF2 may be described as T-tropic, SI, and SF162 as M-tropic, NSI, and are therefore representative of the two major forms of the virus.

Additional work has shown that in keeping with other M-tropic strains of HIV, SF162 is poorly neutralised by both sCD4 and serum from infected patients, and is not very cytopathic in vitro, nor does it down-regulate cell surface CD4 upon infection. Antithetically SF2 is readily neutralised by both sCD4 and serum, is highly cytopathic in vitro, and down-regulates CD4 upon infection (Cheng-Mayer et a l, 1990, Cheng-Mayer

et al, 1989, Koito et al, 1994).

3.1.1.1.1- gpl20 determines the tropism of SF2 and SF162

As reviewed previously in the introduction (see section 1.6.2.3), g p l20 has been shown to be the major determinant of tropism at the viral level. Work on SF2 and SF162 has shown this to be true of these two HIV-1 isolates as well. Recombinant viral DNAs were generated by reciprocal exchange of genetic materials between the two isolates thereby allowing the viral determinants of the cellular host range of SF162 and SF2 to be mapped. Initial studies showed that the env gene was most likely the major determinant of cellular tropism (Cheng-Mayer et a l, 1990), and further work confirmed this and narrowed the area concerned to large parts of the g p l20 portion of the gene (Shioda et a l, 1991), but it was clear that it was not the CD4 binding domain that was important (Liu et a l, 1990).

The two env genes are 89.4% identical at the nucleotide level, which equates to 84.6% at the amino acid level, with most of the differences clustered within the hypervariable regions, V1-V5 (Cheng-Mayer et a l, 1990). Research suggests that the V3 loop is the major determinant of HIV tropism within g p l20 (Hwang et al, 1991). Work on SF2 and SF162 showed that while mutations within the V3 of SF2 lead to a reduction in T-tropism and an increase in M-tropism, in SF162 mutations lead to a reduction in M-tropism, but had only a small effect on its ability to infect T-cell lines (Shioda et a l, 1992). In order to confer full M-tropism on SF2 and T-tropism on SF162 it was necessary to transfer V I, V2

and V3 (Koito et aL, 1994). This implied that the V1/V2 domains interact with V3 to confer on it a more, or less, M-tropic conformation (Koito et aL, 1994, Koito et aL, 1995). Moreover, while mutation of the V3 loop of SF162 conferred a limited ability to replicate in T-cell lines, it did not alter the fact that it remained poorly neutralised by sCD4 (Harrowe & Cheng, 1995), but when the V1/V2 domains were changed, these did confer enhanced sCD4 neutralisation (Koito et aL, 1994).

3.1.1.1.2- Coreceptor usage of SF2 and SF162

That SFl 62 is M-tropic and SF2 is T-tropic suggests that they use CCR5 and CXCR4 respectively as their coreceptors. S F l62 has been shown several times to use only CCR5 and not CXCR4 (Berson et aL, 1996, Cheng-Mayer et aL, 1997, Platt et aL, 1998, Trkola

et aL, 1998, Wang et aL, 1998, Zauli et aL, 1992). However the story for SF2 is less clear. Some have shown that it uses only CXCR4 and not CCR5 (Berson etaL, 1996, Montefiori

et aL, 1998), while others have shown that it can use both (Cheng-Mayer et aL, 1997, Trkola era/., 1998).

CCR5 utilisation has been shown to be dependent on a conserved arginine residue at position 298 within the V3 loop (Wang et aL, 1998), and this is found in both SFl 62 and SF2, which suggests that SF2 might be able to use CCR5. However all the coreceptor usage studies have been carried out in transfected cell lines, which often have artificially high levels of both CD4 and coreceptor. Work has shown that the more CD4 present on the cell surface the less coreceptor required for infection and visa-versa (Platt et aL, 1998), thus if both are at high levels coreceptor usage might occur that is not seen in vivo. Indeed Cheng-Mayer et al have shown that SF2 uses predominantly CXCR4, and that it can only use CCR5 as a coreceptor if levels are artificially high (Cheng-Mayer et aL, 1997). Also it has been shown several times that SF2 is unable to infect and replicate in macrophages (Cheng-Mayer et aL, 1997, Shioda et aL, 1991). Finally while S F l62 and other CCR5- utilising M-tropic strains inhibit the binding of M IP-lp to CCR5, SF2 cannot (Trkola et aL, 1996a).

This suggests that in vitro cell line data for coreceptor usage does not necessarily reflect either coreceptor usage in vivo or in primary cells (Cheng-Mayer et aL, 1997), especially since macrophages and dendritic cells express only low levels of CD4 and CCR5. It also explains why conflicting data is seen for SF2. Therefore one can presume that while

in vitro, circumstances are not clear cut, in vivo SF2 probably can use CXCR4 and not CCR5 while S F l62 uses CCR5 and not CXCR4. Many more coreceptors are now being identified, but they do not appear to be as important, and again they have only been shown to support infection in transfected cell lines, so their in vivo relevance is still unresolved.