Loss of Circulating CD4 T Cells with B Cell Helper Function during Chronic HIV Infection

<div><p>The interaction between follicular T helper cells (T_FH) and B cells in the lymph nodes and spleen has a major impact on the development of antigen-specific B cell responses during infection or vaccination. Recent studies described a functional equivalent of these cells among circulating CD4 T cells, referred to as peripheral T_FH cells. Here, we characterize the phenotype and in vitro B cell helper activity of peripheral T_FH populations, as well as the effect of HIV infection on these populations. In co-culture experiments we confirmed CXCR5+ cells from HIV-uninfected donors provide help to B cells and more specifically, we identified a CCR7^highCXCR5^highCCR6^highPD-1^high CD4 T cell population that secretes IL-21 and enhances isotype-switched immunoglobulin production. This population is significantly decreased in treatment-naïve, HIV-infected individuals and can be recovered after anti-retroviral therapy. We found impaired immunoglobulin production in co-cultures from HIV-infected individuals and found no correlation between the frequency of peripheral T_FH cells and memory B cells, or with neutralization activity in untreated HIV infection in our cohort. Furthermore, we found that within the peripheral T_FH population, the expression level of T_FH-associated genes more closely resembles a memory, non-T_FH population, as opposed to a T_FH population. Overall, our data identify a heterogeneous population of circulating CD4 T cells that provides <i>in vitro</i> help to B cells, and challenges the origin of these cells as memory T_FH cells.</p></div

Progressive loss of pT_FH cells in HIV infection.

<p>(<b>A</b>) Pooled data showing the frequency (%) of CXCR5^high, CXCR5^highCCR6^high and CXCR5^highCCR6^highPD-1^high populations in total CD4 cells from PBMC from HIV uninfected (open circles; n = 13), HIV-infected (treatment-naïve), CD4 count >200 (light gray circles; n = 44), and HIV-infected (treatment-naïve), CD4 count <200 (black circles; n = 22). Significant differences between HIV-uninfected and HIV-infected subjects were determined using the Mann-Whitney U test. ***p<0.001; **p<0.01; *p<0.05. (<b>B</b>) Longitudinal analysis showing the frequency (%) of CXCR5^high, CXCR5^highCCR6^high and CXCR5^highCCR6^highPD-1^high populations in total CD4 cells or indicated populations in CXCR5-expressing cells (bottom row) from HIV-infected (treatment naïve) subjects (n = 10) over 36–48 months. No significant correlations were found. (<b>C</b>) Pooled data showing the frequency (%) of CXCR5^high, CXCR5^highCCR6^high and CXCR5^highCCR6^highPD-1^high populations in total CD4 cells from PBMC from HIV-uninfected subjects (open circles; n = 13) and HIV-infected subjects before (n = 14, week 0; black circles) and after ART (week 24, dark gray circles; week 48, light gray circles). Significant differences between HIV-uninfected and HIV-infected subjects were determined using the Mann-Whitney U test. Significant differences between subjects before and after ART were determined using the Wilcoxon matched-pairs signed rank test. ***p<0.001; **p<0.01; *p<0.05.</p

Characterization of peripheral T_FH cells.

<p>(<b>A</b>) Left: Representative flow cytometry plots from HIV-uninfected PBMC showing the gating scheme for isolating T cell subsets for the T cell/B cell coculture assay. Isolated populations include naïve cells (brown), CM CCR7^low (pink), CM CCR7^highCXCR5^low (orange), CM CCR7^highCXCR5^highCCR6^lowPD-1^high (green), CM CCR7^highCXCR5^highCCR6^highPD-1^low (blue) and CCR7^highCXCR5^highCCR6^highPD-1^high (red). Before gating on CCR6 and PD-1, cells were first gated on CD150^high. Right: Scatter plot indicating the frequency of each population in HIV-uninfected subjects (<i>n</i> = 13). Cells were not gated on CD150 for phenotypic analysis. (<b>B</b>) Indicated CD4 T cell populations were cultured with autologous naïve B cells (CD19^highCD27^lowIgD⁻) in the presence of SEB for 12 days and Ig concentrations were measured from supernatants (n = 6). (<b>C</b>) Indicated CD4 T cell populations were cultured with autologous naïve B cells in the presence of SEB for 2 days and cytokine concentrations were measured from supernatants (n = 6). Horizontal lines indicate limit of detection. Significant differences were determined using the Friedman test with Dunn's multiple comparison post-test. *p<0.05; **p<0.01.</p

Relationship between pT_FH cells and T_FH cells in human tonsil.

<p>(<b>A</b>) Representative flow cytometry plots from HIV-uninfected, pediatric tonsils showing the gating scheme for determining the frequency of CCR6^high cells in T_FH (CXCR5^highPD-1^high) and non-T_FH populations. (<b>B</b>) Bar graphs showing the frequency of CCR6^high cells in T_FH and non-T_FH populations in human tonsils (n = 5). (<b>C</b>) Heatmap analysis of selected genes from RNA-seq data comparing pT_FH cells (CXCR5^highCCR6^highPD-1^high) from HIV-uninfected donors, pT_FH cells from HIV-infected donors, non-T_FH CD4 memory tonsil cells (CM CD57^lowPD-1^dimCCR7^highCCR5^lowCXCR4^low), non-germinal center T_FH tonsil cells (CM CD57^lowPD-1^highCCR7^lowCXCR5^high) and germinal center T_FH tonsil cells (CM PD-1^highCD57^high) from HIV-uninfected donors. (<b>D</b>) Top: Comparison of MAF expression on CD4 T cells from blood or tonsil. Bottom: Geometric mean (MFI) of MAF expression in the indicated populations of central memory CD4 T cells normalized to MAF MFI in naïve CD4 T cells.</p

Functional characteristics of pT_FH cells and the impact of HIV.

<p>(<b>A</b>) Representative flow cytometry plots showing CM, CD154-positive, cytokine-positive cells after SEB stimulation. CD154-positive, cytokine-positive CD4 T cells, shown by contour plots (blue: HIV-uninfected; red: HIV-infected), are overlaid onto 2 dimensional density plots for CM CD4 T cells plotted against CCR7 and CD3, and CXCR5 and CCR6. (<b>B</b>) Bar graphs showing the frequency of SEB-stimulated CD154-positive, cytokine-positive cells that express CCR7, CXCR5 and CCR6 (Blue: uninfected; n = 5; Red: HIV-infected; n = 24). (<b>C</b>) Left: Gag-specific CD4+ T cells (CD154-positive, cytokine-positive) shown as red contour plots are overlaid onto 2 dimensional density plots for CM cells CD4 T cells plotted against CCR7 and CD3, and CXCR5 and CCR6. Right: Bar graphs showing the frequency of Gag-specific CD154-positive, cytokine-positive cells that express CCR7, CXCR5 and CCR6 (n = 14). *p<0.05.</p

Impaired B cell help by pT_FH cells in HIV infection.

<p>(<b>A</b>) CCR7^highCXCR5^low and CCR7^highCXCR5^highCCR6^high CM CD4 T cells isolated from PBMCs were cultured with autologous naïve B cells (CD19^highCD27^lowIgD⁻) in the presence of SEB for 12 days and Ig concentrations were measured from supernatants (HIV-uninfected, n = 8; HIV-infected (non-viremic), n = 5–7, HIV-infected (viremic), n = 1–2). Significant differences were determined using the Wilcoxon paired t-test or the Mann-Whitney test. *p<0.05; **p<0.01. (<b>B</b>) Top: HIV-uninfected PBMCs were incubated with indicated concentrations of CXCL-13 for 1 hour at 37°C (red) or 4°C (black). Bottom: Healthy PBMCs were incubated with 1 µg/mL CXCL13 for 10, 30, 60 or 120 minutes at 37°C (red) or 4°C (black). The frequency of CXCR5-positve CD4 T cells was calculated and normalized to time 0. (n = 3). (<b>C</b>) Top: Correlative analysis showing the frequency of CM CXCR5-positive CD4 T cells versus viral load (n = 27; r = −0.4036, P = 0.0368). Bottom: Correlative analysis showing the concentration of CXCL-13 in plasma or sera versus viral load (n = 27; r = 0.4414, P = 0.0165). Correlations were analyzed using the nonparametric Spearman test.</p