Defective antigen presentation by Mycobacterium tuberculosis-infected monocytes

Area of Science:

• Immunology
• Microbiology
• Cell Biology

Background:

• Human monocytes are crucial for initiating adaptive immune responses by presenting antigens to T cells.
• Mycobacterium tuberculosis (M. tuberculosis) is a pathogen that infects monocytes, potentially disrupting their immune functions.

Purpose of the Study:

• To investigate how in vitro infection with M. tuberculosis affects the antigen-presenting capacity of human monocytes.
• To determine the impact of M. tuberculosis on T-cell activation and monocyte accessory functions.

Main Methods:

• Human monocytes were infected with viable and heat-killed M. tuberculosis at a high bacterium-to-monocyte ratio (50:1).
• Antigen presentation assays were performed using tetanus toxoid as the soluble antigen.
• T-cell proliferation and gamma interferon release were measured.
• Monocyte accessory function was assessed using pokeweed mitogen.
• Expression of human leukocyte antigen DR (HLA-DR) was analyzed.

Main Results:

• M. tuberculosis infection significantly reduced tetanus toxoid-specific T-cell proliferation and gamma interferon release.
• Infected monocytes did not exhibit suppressive activity towards T cells stimulated by control monocytes.
• Fixed M. tuberculosis-infected monocytes lost their antigen-presenting capacity.
• Uptake of M. tuberculosis reduced HLA-DR expression on monocytes.
• Monocytes infected with M. tuberculosis showed reduced efficiency as accessory cells.

Conclusions:

• High-dose M. tuberculosis infection impairs the ability of human monocytes to process and present soluble antigens.
• M. tuberculosis-infected monocytes are less effective in supporting T-cell activation due to impaired antigen presentation and accessory functions.