Sildenafil, a Type-5 Phosphodiesterase Inhibitor, Fails to Reverse Myeloid-Derived Suppressor Cell-Mediated T Cell Suppression in Cells Isolated From Tuberculosis Patients

Abstract

Successful TB treatment is hampered by increasing resistance to the two most effective first-line anti-TB drugs, namely isoniazid and rifampicin, thus innovative therapies focused on host processes, termed host-directed therapies (HDTs), are promising novel approaches for increasing treatment efficacy without inducing drug resistance. We assessed the ability of Sildenafil, a type-5 phosphodiesterase inhibitor, as a repurposed compound, to serve as HDT target, by counteracting the suppressive effects of myeloid-derived suppressor cells (MDSC) obtained from active TB cases on T-cell responsiveness. We confirm that MDSC suppress non-specific T-cell activation. We also show that Sildenafil treatment fails to reverse the MDSC-mediated suppression of T-cell functions measured here, namely activation and proliferation. The impact of Sildenafil treatment on improved immunity, using the concentration tested here, is likely to be minimal, but further identification and development of MDSC-targeting TB host-directed therapies are warranted.

Keywords: Mycobacterium tuberculosis; Sildenafil; host-directed therapies; myeloid-derived suppressor cells; tuberculosis.

Figure 1

Effect of MDSC on Helper T cell and Helper T cell subset responses. Frequency of Helper T cells (CD3+CD4+) and Helper T cell subsets from in vitro T cell co-culture of autologous T cells (CD3+ cells), MDSC (HLA-CD33+ cells) and control monocytes (HLA+ cells) isolated from PBMC of individuals with active TB disease (n = 7). Plots show individual T cell replicates. Panels (A-D) D’Agostino & Pearson test measured normality. Repeated measures one way ANOVA with Sidak’s multiple comparisons test (parametric data) and Friedman test (non-parametric data). P<0.05 was considered significant.

Figure 2

Effect of Sildenafil on the frequency of (A) Total MDSC, (B) M-MDSC, (C) PMN-MDSC, (D) Control Monocytes, gated as a frequency of MACS isolated HLA-CD33+ or HLA+ cells, from in vitro cultures of MDSC (HLA-CD33+ cells) and Control Monocytes isolated from PBMC of individuals with active TB disease (n = 7). Plots show individual TB patient data. D’Agostino and Pearson normality Test. Paired t test (parametric data) and Wilcoxon matched pairs signed rank test (non-parametric data). Median with interquartile range indicated on plots. P<0.05 was considered significant.

Figure 3

Effect of Sildenafil on the frequency of (A) Total MDSC, (B) M-MDSC, (C) PMN-MDSC, gated as a frequency of PBMC, from in vitro cultures of MDSC (HLA-CD33+ cells) and Control Monocytes isolated from PBMC of individuals with active TB disease (n = 7). Plots show individual TB patient data. D’Agostino and Pearson normality Test. Paired t test (parametric data) and Wilcoxon matched pairs signed rank test (non-parametric data). Median with interquartile range indicated on plots. P<0.05 was considered significant.

Figure 4

Sildenafil supplementation does not affect T cell or MDSC cytokine production (IL2, TNFb, GMCSF, IFNg, TNFa). Analyte concentration (pg/ml) was measured for various culture combinations (MDSC only, MDSC-T cell coculture, and T cell only), under differing stimulation combinations (Unstimulated and anti-CD3CD28 bead Activation) with or without Sildenafil treatment, by multiplex analysis. Plots show analyte concentration in culture supernatant. One-way ANOVA with Tukey post-hoc correction. Median with interquartile range indicated on plots. P<0.05 was considered significant. Pairwise comparisons which were ns are not shown on plot.