PD-1 Expression on Mycobacterium tuberculosis-Specific CD4 T Cells Is Associated With Bacterial Load in Human Tuberculosis.

Cheryl L Day,Gerhard Walzl,Lynnett Stone,Wendy A Burgers,Marwou De Kock,Rubina Bunjun,Deborah A Abrahams,Robert J Wilkinson,Willem A Hanekom

doi:10.3389/fimmu.2018.01995

Abstract

Persistent antigen stimulation in chronic infections has been associated with antigen-specific T cell dysfunction and upregulation of inhibitory receptors, including programmed cell death protein 1 (PD-1). Pulmonary tuberculosis (TB) disease is characterized by high levels of Mycobacterium tuberculosis (Mtb), yet the relationship between bacterial load, PD-1 expression, and Mtb-specific T cell function in human TB has not been well-defined. Using peripheral blood samples from adults with LTBI and with pulmonary TB disease, we tested the hypothesis that PD-1 expression is associated with bacterial load and functional capacity of Mtb-specific T cell responses. We found that PD-1 was expressed at significantly higher levels on Th1 cytokine-producing Mtb-specific CD4 T cells from patients with smear-positive TB, compared with smear-negative TB and LTBI, which decreased after completion of anti-TB treatment. By contrast, expression of PD-1 on Mtb-specific CD8 T cells was significantly lower than on Mtb-specific CD4 T cells and did not differ by Mtb infection and disease status. In vitro stimulation of PBMC with Mtb antigens demonstrated that PD-1 is induced on proliferating Mtb-specific CD4 T cells and that Th1 cytokine production capacity is preferentially maintained within PD-1+ proliferating CD4 T cells, compared with proliferating Mtb-specific CD4 T cells that lack PD-1 expression. Together, these data indicate that expression of PD-1 on Mtb-specific CD4 T cells is indicative of mycobacterial antigen exposure and identifies a population of effector cells with Th1 cytokine production capacity. These studies provide novel insights into the role of the PD-1 pathway in regulating CD4 and CD8 T cell responses in Mtb infection and provide rationale for future studies to evaluate PD-1 expression on antigen-specific CD4 T cells as a potential biomarker for bacterial load and treatment response in human TB.

Highlights

Infection with Mycobacterium tuberculosis (Mtb) is responsible for over 10 million cases of tuberculosis (TB) and approximately 1.7 million deaths each year [1]
On day 6 of the assay, cells were re-stimulated with a CFP-10/ESAT-6 peptide pool for 6 h, and analyzed by flow cytometry for expression of PD-1 and intracellular IFN-γ and TNF-α. (A) Flow cytometry data indicating IFN-γ production capacity by proliferating
Plots are shown gated on viable CD3+CD4+ lymphocytes; data are shown from an individual with latent Mtb infection (LTBI) and a patient with smear+ TB. (B) Flow cytometry data indicating co-expression of PD-1 and IFN-γ by OGlo CD4 T cells following stimulation with a CFP-10/ESAT-6 peptide pool

Summary

INTRODUCTION

Infection with Mycobacterium tuberculosis (Mtb) is responsible for over 10 million cases of tuberculosis (TB) and approximately 1.7 million deaths each year [1]. Mtb infection of PD-1−/− mice leads to increased frequencies of Mtb-specific CD4 T cells; PD-1−/− mice display enhanced susceptibility to TB disease, characterized by increased bacterial loads, increased inflammatory and necrotic responses in the lungs, and reduced survival [34,35,36]. These studies in PD-1-deficient mice demonstrate a necessary role for PD-1 in limiting excessive IFN-γ production by CD4 T cells, which has been associated with exacerbated disease in murine models of TB [37]. To test the hypothesis that PD-1 expression is associated with bacterial load and Mtb-specific T cell functional capacity, we conducted a comprehensive analysis of PD-1 expression on Mtb-specific CD4 and CD8 T cells in clinically well-characterized cohorts of South African adults across a spectrum of Mtb infection and disease

MATERIALS AND METHODS

Ethics Statement

Participants

DISCUSSION