Studies of immune response in human tuberculosis

Abstract

TB (tuberculosis), caused by Mycobacterium tuberculosis (Mtb), continues to be a world-leading killer and a serious global health problem primarily affecting poor people in many developing countries. The difficult situation with TB/HIV co-infection is also a major key challenge to public health. Despite recent advances in TB research, the host- and pathogen-specific factors that lead to protective immunity, particularly in humans, remain unclear. While it is well-established that cell-mediated immunity is required to control TB infection, the role of humoral immunity including Th2 immune responses is debated. This thesis aimed to explore immune responses in human TB and the immunopathogenic mechanisms involved in the progression of clinical TB in both HIV-negative and HIV-positive individuals.

To address the aims of this thesis work, well-defined study cohorts of active TB patients were obtained in close collaboration with the Black Lion University Hospital in Addis Ababa, Ethiopia. We used a novel immunodiagnostic test, Antibodies in Lymphocyte Supernatants (ALS), to demonstrate that IgG-secreting plasmablasts were significantly higher in the peripheral circulation of patients with active TB compared to latent TB cases and non-TB controls. Interestingly, BCG-specific IgG titers were particularly high in blood samples from TB/HIV co-infected patients with CD4 T cell counts <200 cells/ml who produced low levels of Mtb-specific IFN-γ in vitro. A technological platform including quantitative assessments of mRNA and protein expression in tissue (lymph nodes), fluids (bronchoalveolar lavage (BAL) and pleura fluid) and peripheral blood, was also used to investigate antimicrobial effector pathways and adverse immune responses in unique clinical samples obtained from the local site of Mtb infection.

The results from these studies revealed that TB disease was associated with extensive tissue remodelling including an altered cellular composition, collagen deposition and granuloma formation. Here, the degree of necrotic granuloma formation was particularly prominent in TB/HIV-co-infected patients. Despite granuloma enrichment of activated CD68+ macrophages containing Mtb-antigens, mRNA levels of IFN-γ, TNF-α and IL-17 remained low in Mtb-infected lymph nodes. Accordingly, CD8+ T cells expressing cytolytic and antimicrobial effector molecules perforin and granulysin were low inside the TB lesions, while CD4+FoxP3+ regulatory T cells (Treg) and Th2/Treg cell cytokines IL-13 and TGF-β were up-regulated in the Mtb-infected tissues. The observed shift of the immune response from a Th1/Th17 towards an immunoregulatory phenotype was supported by our finding of a Th2 polarized response in the lung of patients with pulmonary TB. Here, multiplex protein analysis of BAL and plasma samples demonstrated low levels of Th1/Th17 cytokines and the T cell-chemoattractant CCL5, but significantly up-regulated levels of pro-inflammatory cytokines and the Th2 cytokine IL-4. The enhanced Th2 response was associated with increased levels of CCL4, suppressors of cytokine signaling-3 (SOCS3) and mycobacteria-specific IgG in BAL fluid from patients with active pulmonary TB. Contrary, IL-4, CCL4, SOCS3 and IgG-responses remained low in patients with less severe extrapulmonary pleural TB disease, who demonstrated up-regulated levels of both IFN-γ and CCL5.

Taken together, our results provide evidence that human TB is associated with impaired Th1 immunity but elevated Th2/immunoregulatory responses and induction of antibody-mediated immunity. Importantly, enhanced Th2 and/or plasmablast responses may be used as relevant biomarkers or immune response signatures of active progressive TB disease that could be explored as potential targets for clinical TB management in the future.