Antigens for CD4 and CD8 T cells in tuberculosis

Understanding and overcoming the barriers to T cell-mediated immunity against tuberculosis

Despite the overwhelming success of immunization in reducing, and even eliminating, the global threats posed by a wide spectrum of infectious diseases, attempts to do the same for tuberculosis (TB) have failed to date. While most effective vaccines act by eliciting neutralizing antibodies, T cells are the primary mediators of adaptive immunity against TB. Unfortunately, the onset of the T cell response after aerosol infection with Mycobacterium tuberculosis (Mtb), the bacterium that causes TB, is exceedingly slow, and systemically administered vaccines only modestly accelerate the recruitment of effector T cells to the lungs. This delay seems to be orchestrated by Mtb itself to prolong the period of unrestricted bacterial replication in the lung that characterizes the innate phase of the response. When T cells finally arrive at the site of infection, multiple layers of regulation have been established that limit the ability of T cells to control or eradicate Mtb. From this understanding, emerges a strategy for achieving immunity. Lung resident memory T cells may recognize Mtb-infected cells shortly after infection and confer protection before regulatory networks are allowed to develop. Early studies using vaccines that elicit lung resident T cells by targeting the lung mucosa have been promising, but many questions remain. Due to the fundamental nature of these questions, and the need to understand and manipulate the early events in the lung after aerosol infection, only coordinated approaches that utilize tractable animal models to inform human TB vaccine trials will move the field toward its goal.

Helminth infections coincident with active pulmonary tuberculosis inhibit mono- and multifunctional CD4+ and CD8+ T cell responses in a process dependent on IL-10

Tissue invasive helminth infections and tuberculosis (TB) are co-endemic in many parts of the world and can trigger immune responses that might antagonize each other. We have previously shown that helminth infections modulate the Th1 and Th17 responses to mycobacterial-antigens in latent TB. To determine whether helminth infections modulate antigen-specific and non-specific immune responses in active pulmonary TB, we examined CD4⁺ and CD8⁺ T cell responses as well as the systemic (plasma) cytokine levels in individuals with pulmonary TB with or without two distinct helminth infections—Wuchereria bancrofti and Strongyloides stercoralis infection. By analyzing the frequencies of Th1 and Th17 CD4⁺ and CD8⁺ T cells and their component subsets (including multifunctional cells), we report a significant diminution in the mycobacterial–specific frequencies of mono- and multi–functional CD4⁺ Th1 and (to a lesser extent) Th17 cells when concomitant filarial or Strongyloides infection occurs. The impairment in CD4⁺ and CD8⁺ T cell cytokine responses was antigen-specific as polyclonal activated T cell frequencies were equivalent irrespective of helminth infection status. This diminution in T cell responses was also reflected in diminished circulating levels of Th1 (IFN-γ, TNF-α and IL-2)- and Th17 (IL-17A and IL-17F)-associated cytokines. Finally, we demonstrate that for the filarial co-infections at least, this diminished frequency of multifunctional CD4⁺ T cell responses was partially dependent on IL-10 as IL-10 blockade significantly increased the frequencies of CD4⁺ Th1 cells. Thus, co-existent helminth infection is associated with an IL-10 mediated (for filarial infection) profound inhibition of antigen-specific CD4⁺ T cell responses as well as protective systemic cytokine responses in active pulmonary TB.

While it has long been recognized that helminth infections alter the pathophysiology of allergic and autoimmune disease, data suggest that helminth infections also exert an important immunological effect on concomitant infections and vaccine responses. In particular, helminth coinfection can modulate the severity, pathogenesis and transmission of other infectious diseases. In this study, we examine the mechanism by which helminth infections modulate the immunological responses to tuberculosis antigens in individuals with active pulmonary tuberculosis. Our data suggest that two different helminth infections, with different life cycles, tissue localization and modes of transmission essentially exert very similar effects on the adaptive immune response to tuberculosis antigens in pulmonary tuberculosis. This includes a compromised induction of protective cytokine-expressing T cells as well as inhibitory effects on systemic cytokines that are potentially protective in tuberculosis. The strength of this study lies in the fact that this is the first study to demonstrate that two different helminth infections essentially impair cytokine responses in a similar manner in pulmonary tuberculosis.