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Department of Microbiology and Immunology, Cornell University, Weill Medical College, New York, New York
To the EditorKrathwohl and Kaiser reported that recombinant gp120, the surface glycoprotein of human immunodeficiency virus type 1 (HIV-1), inhibits the proliferation of presumably CD4- human neural progenitor cells in vitro [1]. They attributed this phenomenon to intracellular signaling caused by the interaction between gp120 and cell-surface chemokine receptors (CXCR4 and/or CCR3 but not CCR5). Cerebrospinal fluid (CSF) from patients with AIDS dementia had similar effects on the proliferation of neural progenitor cells, which Krathwohl and Kaiser concluded were due to gp120 in the fluid. We have reservations about this study, which are based on knowledge of plausible in vivo concentrations of gp120, its antigenic properties, and its affinity for chemokine receptors.
Krathwohl and Kaiser used 3 different gp120 proteins: 1 derived from HIV-1 IIIb, which is a T-cell lineadapted, CXCR4-using (X4 strain) virus from subtype B, and 1 each derived from HIV-1 CM235 and HIV-1 93TH975, which are primary CCR5-using (R5 strain) viruses from Env subtype E (http://www.hiv.lanl.gov). The gp120 derived from the IIIb and CM235 strains inhibited the proliferation of neural progenitor cells; the gp120 derived from the 93TH975 strain had no effect. The 3 gp120 proteins were used at concentrations of 0.81 nmol/L (100 ng/mL). The inhibitory effects of the gp120 derived from the IIIb and CM235 strains and of CSF were abrogated by a murine monoclonal antibody (MAb) to the V3 region of gp120. The "cross-reactive" and "broadly neutralizing" MAb [1, pp. 217, 221] was from Immunodiagnostics, although no specific product code was reported. Immunodiagnostics sells 2 murine MAbs to the V3 region of gp 120, 1101 and 1121.
It would be surprising if a murine MAb to the V3 region of gp120 of the IIIb strain cross-reacted with the V3 region of the subtype E strain CM235, because the V3 sequences of gp120 of the IIIb and CM235 strains are widely divergent (figure 1; http://www.hiv.lanl.gov).
We investigated the reactivities of MAbs 1101 and 1121 with gp120 of the CM235 and IIIb strains, obtained from Protein Sciences and Immunodiagnostics, respectively [1]. We tested these and several other gp120 proteins (HXB2, JR-FL, ADA, and BaL from subtype B; DU151 from subtype C; and Q23 from subtype A) in immunoassays based on capture of gp120 by the anti-C5 antibody D7324 [2, 3]. CD4-IgG2, used as a positive control [4], bound to all types of gp120, including that of the CM235 strain. MAb 1121 reacted efficiently with gp120 of the IIIb, HXB2, JR-FL, and ADA strains but not with that of the BaL strain or with that of any nonsubtype B strain, even at a concentration of 3 g/mL. MAb 1101 bound to gp120 of the ADA and BaL strains, but not to those of the IIIb, HXB2, JR-FL, or nonsubtype B strains. HXB2 is a clone of the IIIb isolate, which is closely related to the LAI virus. MAb 1101 is reported to bind to gp120 of the IIIb strain (http://www.immunodx.com). However, we did not observe binding of MAb 1101 either to gp120 of the HXB2 clone or to Immunogenetics's gp120 of the IIIb strain. Single amino-acid variations in the V3 regions of gp120 of different clones of IIIb and related viruses can affect the binding of some MAbs [3].
We cannot, therefore, explain how either of these V3-specific MAbs could reverse the effects that gp120 of the CM235 strain has on neural progenitor cells. Moreover, because of their limited cross-reactivity, it would be surprising if either MAb reacted with most of the gp120 proteins present in CSF samples from HIV-1infected patients, given the antigenic diversity of primary viruses (http://www.hiv.lanl.gov). It is difficult to understand how a V3-specific MAb could counteract the effects of gp120 proteins to which it does not bind.
The concentrations of gp120 that were used by Krathwohl and Kaiser, 0.8 and 1.0 nmol/L, are 40500 times the plausible range in plasma [5]. The number of copies of viral RNA in CSF tends to be lower than that in plasma [6, 7]. In the CSF samples from patients with or without dementia, the viral load was, on average, 23 log10 RNA copies of RNA/mL, although only CSF (used at a 50-fold dilution) from the former group inhibited the proliferation of neural progenitor cells in vitro [1]. Thus, the cells may have been exposed to only 2 × 10-19 mol/L of gp120, if RNA and gp120 were present at typical concentration ratios [5]. Yet, the CSF was as inhibitory as recombinant gp120 at a concentration that was 10 orders of magnitude higher [1].
The argument that the effects of gp120 of X4 strains are mediated by means of CXCR4, and those of gp120 of R5 strains by means of CCR3 (not CCR5), is also problematic [8]. Even at a concentration of 1 nmol/L, most gp120 proteins bind to chemokine receptors, in the absence of CD4, to a negligible extent [5]. The receptor occupancy by any gp120 present at many orders of magnitude lower concentrations in diluted CSF would be infinitesimal. Krathwohl and Kaiser suggest that the interaction between gp120 and cell-surface heparan-sulfate proteoglycans (HSPGs) facilitates its subsequent binding to chemokine receptors; however, although HSPGs interact strongly with gp120 of X4 strains, they do so poorly with gp120 of R5 strains [9], such as CM235.
Whether HIV-1induced dementia is caused directly by viral proteins that act on neural cells or indirectly by the activation of chemokine- and cytokine-secreting cells [10] is a complex issue beyond the scope of this letter. But, even if all the necessary experimental controls are performed, which they often are not, the addition of gp120 to neural progenitor cells in vitro does seem unlikely to mimic all the biological processes associated with HIV-1 infection of the brain.
References
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