In a correlative study, the mean plasma level of the chemokine stromal-cell-derived factor 1 (SDF-1) was lower in subjects with syncytium-inducing (SI) than in subjects with non-syncytium-inducing (NSI) HIV isolates, regardless of the CD4 cell count or when compared with HIV-negative individuals. Individuals with high SDF-1 had an 81% probability of having an NSI virus phenotype compared with individuals with lower SDF-1. Increased expression of SDF-1 may help explain why the more pathogenic SI HIV-1 variants do not appear in some individuals.
Primary HIV-1 infection usually starts with non-syncytium-inducing (NSI) variants that use the chemokine receptor CCR5 (R5 variants). Later during infection, syncytium-inducing (SI) HIV-1 variants, which use CXCR4 as co-receptor (X4 variants), emerge in half the cases. The SI phenotype has been associated with faster T cell decline, immune failure, and the onset of AIDS . These findings suggest that the HIV-1 phenotype switch is a causal factor of AIDS , and that host and not viral factors drive and contain the emergence of SI/X4 variants. However, it remains unclear why SI/X4 variants emerge.
We have shown that a CXCR4 antagonist, in cell culture infections, prevents the emergence of SI HIV-1 variants . A polymorphism in the promoter region of the stromal-cell-derived factor 1 (SDF-1) gene has been associated with an effect on disease progression [4,5], and the constitutive expression of SDF-1 has been thought to be a plausible explanation for the control of HIV-1 X4 transmission and propagation [6,7]. SDF-1, the natural ligand of CXCR4, may thus have an in-vivo effect on the availability of CXCR4 for HIV-1.
Plasma samples from 42 HIV-negative volunteers (control group) and 172 HIV-positive individuals were evaluated for the levels of SDF-1 in plasma by an in-house enzyme-linked immunosorbent assay performed as described [8,9]. The expression of CXCR4 in the corresponding peripheral blood mononuclear cells (PBMC) was followed by flow cytometry in HIV-negative or an available subset of HIV-positive individuals.
The mean SDF-1 concentration in the control group was 419 ± 282 pg/ml whereas the mean CXCR4 expression was 90 ± 4% of total lymphocytes with a mean fluorescence intensity (MFI) of 75 ± 18. The level of SDF-1 in the plasma significantly correlated with the percentage of PBMC expressing CXCR4 (P < 0.05), although the association was weak (r = −0.34).
Standard virological and immunological markers were evaluated in the HIV-positive group. An inverse correlation was found between the CD4 cell count and viral load (r = −0.58, P < 0.01). The mean CD4 and CD8 cell counts were 776 ± 551 and 398 ± 434, respectively. The mean viral load was 5.25 ± 5.47 log10 copies/ml.
PBMC from each patient were co-cultured with PBMC from HIV-negative donors to isolate virus. HIV-1 p24 antigen-containing supernatant from each co-culture was then used for evaluation of the SI phenotype in MT-2 cells. A total of 79 virus samples tested as SI and 93 as NSI. Patients from these two groups had significantly different (P < 0.001) mean CD4 and CD8 cell counts (150 ± 178 and 320 ± 223 CD4 cells/μl and 663 ± 452 and 849 ± 597 CD8 cells/μl for the SI and NSI groups, respectively) despite a little (less than 0.5 log units) but significant difference in viral load (5.45 ± 5.58 and 5.01 ± 5.2 for the SI and NSI groups, respectively).
There were no significant differences in SDF-1 values between the control group and the HIV-positive group (mean SDF-1 419 ± 282 and 398 ± 434 pg/ml, respectively). The group of SI samples had a significantly lower mean SDF-1 (P < 0.05) than the NSI group (319 ± 206 and 467 ± 549 pg/ml for the SI and NSI groups, respectively). Fig. 1a shows the SDF-1 levels of HIV-positive individuals when plotted in groups according to the CD4 cell count and the SI and NSI phenotype. The NSI subsets always had higher SDF-1 levels than the SI subsets, regardless of the CD4 cell count. However, there were no significant differences between mean SDF-1 concentrations among the NSI subsets or among the SI subsets in the three CD4 cell groups. The mean SDF-1 levels thus remained unchanged despite immunological failure (CD4 cell count < 200 cells/μl), but were consistently higher in the NSI subset. Nevertheless, the NSI subsets showed a relatively higher SD, which may reflect the existence of NSI individuals with low SDF-1 that have not yet converted to the SI phenotype. Consistently, individuals with CD4 cell counts lower than 200 cells/μl had a 66% probability of having SI variants. This probability decreased to 24 or 17% when the CD4 cell count ranged from 200 to 500 cells/μl or over 500 cells/μl, respectively.
When HIV-positive individuals were grouped according to SDF-1 levels, of the 26 HIV-positive individuals with high (> 570 pg/ml) SDF-1, 21 (81%) had the NSI and five (19%) the SI phenotype (Fig. 1b). The percentage of SI individuals increased to 43 and 56% in the intermediate (SDF-1 250–570 pg/ml) and low (SDF-1 < 250 pg/ml) groups, respectively. When we evaluated the expression of CXCR4 in PBMC from a subset of these high SDF-1 individuals (SDF-1 concentration from the subset: 1671 ± 1338 pg/ml, n = 8), a significant (P < 0.05) lower mean CXCR4 expression was found (49 ± 24 MFI) when compared with a subset of individuals with low SDF-1 (SDF-1 < 250 pg/ml, CXCR4 MFI 71 ± 39, n = 13) or intermediate SDF-1 (between 250 and 570 pg/ml; CXCR4 MFI 85 ± 50, n = 15).
Our data show a significant number of individuals with NSI isolates having less than 200 CD4 cells/μl (n = 26). In the NSI subset of less than 200 CD4 cells/μl (n = 26), 30% (eight patients) had SDF-1 levels greater than 570 pg/ml, whereas the remaining 70% (18 patients) had lower SDF-1. Conversely, in the SI subset of less than 200 CD4 cells/μl, only two patients (4%, n = 51) had high SDF-1 levels. We have also shown that high SDF-1 in the plasma modified CXCR4 expression in PBMC, supporting the hypothesis that high SDF-1 will limit the emergence of the SI phenotype, and partly explaining why the SI phenotype does not develop in all patients who progress to AIDS.
SDF-1 levels remain unchanged over time in healthy individuals . Our results show that the SDF-1 level in the plasma is an independent factor of CD4 cell count, viral load, immunological status or presence of the SDF-1 3'A variant (data not shown).
Our data suggest that high SDF-1 levels in the plasma will reflect a predisposition for HIV to retain the NSI phenotype in vivo.
José A. Esté
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