Mycobacterial Antigen-specific T Cell Immune Responses In Patients With Tuberculosis

Tuberculosis (TB), caused by Mycobacterium tuberculosis, is one of the most deadly infectious diseases in the world. It is estimated that 9.27 million new cases of TB occurred in 2007 and 4.1 million were smear-positive. In 2007 alone, there were estimated 1.32 million deaths from TB in HIV-negative people and 456,000 deaths from TB in HIV-positive people. Long-term epidemiological studies indicate that only 5–10% of people infected with M. tuberculosis eventually develop active TB in their lifetime. This indicates that our immune system has the ability to develop a protective immunity that can effectively contain replication of M. tuberculosis inside the body and prevent development into active disease.Protective immunity against TB mainly depends on CD4⁺ T cells and CD8⁺ T cells.??T cells and CD1-restricted T cells also play roles in protection against M. tuberculosis infection. The role of CD4 T cells in protective immunity against TB is largely determined by knockout mice infection experiments, and also by observations that AIDS patients are highly susceptible to development of active TB. Infection of CD4-knockout (CD4KO) mice demonstrated the importance of CD8 T cells in protection as well. CD4KO mice are well protected from M. tuberculosis infection at weeks 6 and 12 post vaccination, but fail to protect at earlier time point. CD8⁺ T cells have direct cytotoxic effect against infected cells and can also produce cytokines. It is generally believed that cytokines produced by Th1 cells, such as IFN-?and TNF-?, are central for protection. Activation of infected macrophage by cytokines leads to restriction of mycobacterial replication and killing of M. tuberculosis inside the host cells.CD27 is a member of the tumor necrosis factor receptor family, has an important role in generation of T-cell immunity. In order to investigate whether CD27 can be used as an immunological marker for diagnosis of active TB and monitoring TB activity in high-burden countries, we used CD154 (also called CD40L) as activation marker to identify CD4⁺ mycobacterial antigen-specific T cells in this study. CD154 is absent from resting CD4⁺ T cells but transiently expressed in activated CD4⁺ T cells. This method does not depend on cytokine secretion or major histocompatibility complex haplotype, and can detect a broad range of antigen-specific CD4⁺ T cells, whether it is cytokine dependent or independent.We studied the CD27 expressing on antigen-specific T cells in patients with new and persistent active pulmonary TB. Compared with tuberculin-positive controls, patients with bacterial culture-positive pulmonary TB had significantly reduced CD27 expression on antigen-specific CD4 T cells(P=0.032). The persistent active TB patients had much lower percentages of CD27⁺ antigen-specific CD4 T cells than culture-positive new TB patients (P=0.008) and healthy controls (P=0.005), while no significant difference was observed among culture-positive new TB patients and healthy controls. CD27 expression on antigen-specific CD4 T cells was not significantly influenced by age. Functional analysis demonstrated that CD27⁺ antigen-specific CD4 T cell subset had significantly more Ki-67+ cells than CD27^- T-cell subset (P=0.016). This result indicates that CD27⁺ antigen-specific CD4⁺ T-cell subset could proliferate significantly upon antigen restimulation. CD27⁺ T cells had more IFN-?⁺ T cells than CD27? T cells (P=0.005). Logistic regression analysis on frequencies of CD27^-expressing antigen-specific CD4 T cells and TB patients' clinical characteristics indicated that low percentage of CD27⁺ antigen-specific CD4 T cells correlated significantly with persistent active tuberculosis (P=0.002, odds ratio = 19.6). In summary, patients with persistent active TB had significantly lower frequencies of CD27⁺ antigen-specific CD4 T cells than culture-positive new TB patients and healthy controls.Memory T cells are hallmark of acquired immunological responses. Most of antigen-specific T cells are short-lived and die after resolution of infection. Memory T cells are long-lived cells that reactivate upon antigen stimulation. Based on surface expression of CD45RA and CCR7, T cells can be divided into CD45RA^-CCR7⁺ central memory T cells (TCM), CD45RA^-CCR7^- effector memory T cells (TEM), and CD45RA⁺CCR7^- effector memory T cells (TEMRA). TCM are long-lived T cell with strong proliferative capacity and are capable of self-renewal. TEM are memory T cells with effector phenotypes and can respond rapidly to re-infection. Current model suggests that TCM might contribute to the maintenance of TEM pool. Understanding the mechanism of protective immunity, especially the role of different T cell subsets in immunity against M. tuberculosis, will lead to new strategy in vaccine design and immunotherapy of tuberculosis.We studied the production and frequencies of M. tuberculosis antigen-specific CD8⁺ memory T cells from 24 patients with pulmonary tuberculosis and 22 individuals with latent infection. CD8⁺ antigen-specific T cells were detected based on surface CD107a expression following antigen stimulation. Analysis of antigen-specific CD8 memory T cells from 24 patients with active pulmonary tuberculosis and 22 individuals with latent infection showed that the frequencies of the CD45RA-positive CD8⁺ effector memory T cell (TEMRA) subset were significantly increased (59.94±3.87% vs 46.65±2.88%,P=0.0203) and the frequencies of CD8⁺ effector memory T cell (TEM) subset were significantly decreased (38.14±3.87% vs 52.34±2.95%,P=0.0142) in patients with active pulmonary TB, compared with individuals with latent infection. The results indicated there are significant difference in frequencies of antigen-specific CD8⁺ T_EMRA and CD8⁺ T_EM among people with active pulmonary TB and latent infection. It may correlate with antigen load in vivo and antigen specific cellular immune responses.Myeloid-derived suppressor cells (MDSCs) play a key role in inhibition of acquired immune responses. Our study investigated the production and frequencies of myeloid-derived suppressor cells (MDSCs) in patients with pulmonary tuberculosis and individuals with latent infection by flow cytometry. Totally 78 pulmonary TB patients, 30 latent infection individuals and 66 healthy controls were studied. Our data indicated that patients with active pulmonary TB had significantly elevated frequencies of CD14^-HLA-DR^-CD33⁺CD11b^high MDSCs compared with individuals with latent infection (P=0.023 and healthy controls (P=0.001). However, CD14⁺HLA^-DR^- MDSCs did not show significant difference among the 3 groups. Serum TGF-?level in TB patients was significantly higher than healthy controls. The results indicated that in active pulmonary TB patients, the frequencies of CD14-HLA-DR-CD33+CD11bhigh MDSCs increased significantly and might play an important role in TB development through suppression of anti-TB immune responses.