Intracellular Bacteria and the Host's Immune System
Intracellular bacteria can bypass or subvert the host's immune system in a number of different ways. Cell-mediated immunity is the primary host defense mechanism against intracellular microorganisms (CMI). Some intracellular bacteria mask their antigens from the opsonizing antibodies in order to avoid being immunely responsive. They coat their bodies in the host's protein components, such as immunoglobulin, fibronectin, or fibrin molecules. This gives them the ability to conceal their antigenic surface components from the host's immune system. By remaining inside the infected cells and preventing the development of the microbial Ag on the surfaces of the host cells, intracellular pathogens can also avoid immune responses. That is evident in cases whereby M. leprae or Brucella, Listeria infect macrophages. The macrophages promote the growth of the intracellular bacteria and also protect them from immunological defenses (Todar, 2012).
Invisible Intracellular Bacteria
Some intracellular bacteria such as E. coli, Listeria, Shigella, and Yersinia do not display their antigens on the surface of infected cells and stay inside them (Todar, 2012). That makes them virtually invisible to the host's immunological defense mechanisms. Some intracellular bacteria also avoid immune response by changing their antigens periodically, for example, undergoing antigenic variation. Indeed, the antigens of intracellular bacteria can vary or change upon the infection of host cells, or the bacteria can have multiple antigenic types (serovars or serotypes). Antigenic variation is a vital mechanism that these bacteria use to escape from the antibodies' neutralizing activities (Reddick & Alto, 2014)
How CD8 and Natural Killer Cells Kill Intracellular Bacteria
CD8 cells, also known as cytotoxic T cells, refer to transmembrane glycoproteins, which serve as co-receptors for T cell receptor (TCR). CD8 cells take notice of the peptides present in MHC Class I molecules (present in all nucleated cells) and bind to their β portion. Conversely, natural killer (NK) cells are cytotoxic lymphocytes that play a vital role in the host's innate immunological defense against intracellular pathogens such as bacteria and viruses (Vivier, Raulet, Moretta, & Caligiuri, 2012). The role of CD8 cells in the immune response is similar to the role of NK cells. They provide prompt responses to intracellular bacterial infections by killing these bacteria and destroying infected and abnormal cells.
Mechanisms of Killing
When CD8 and NK cells recognize intracellular bacteria and become activated, they kill them using various mechanisms. The first one involves the production of cytokines, primarily IFN-γ and TNF-α that have antiviral and antitumor microbial impacts. The second mechanism involves the secretion of cytotoxic granules, which contain two types of proteins, granzymes, and perforin. Granzymes refer to serine proteases that are responsible for cleaving of the proteins inside host cells thus hindering the release of viral proteins and eventually killing the target intracellular bacteria and causing osmotic cell lysis. Perforin, on the other hand, triggers the formation pores on the cell membranes of the target intracellular bacteria through the creation of an aqueous channel. The pores make it possible for the granzymes, also found in cytotoxic granules to penetrate into the malignant or infected cells and kill them (Wissinger, 2016). The third mechanism is through Fas/FasL interactions. Activated CD8 cells express FasL molecules on the target intracellular bacteria, to bind to Fas molecules on the target cells thus causing the Fas molecules to trimerize and pull together signaling molecules, which trigger the activation of the caspase cascade. That induces apoptosis and also kills the target intracellular bacteria (Wissinger, 2016).
Diagram: CD8 and NK Cells Killing Intracellular Bacteria
References
Reddick, L. E., & Alto, N. M. (2014). Bacteria Fighting Back – How Pathogens Target and Subvert the Host Innate Immune System. National Center for Biotechnology Information, 54(2), 321–328.
Todar, K. (2012). Bacterial Defense against Specific Immune Responses. Madison, Wisconsin: The American Society for Microbiology.
Vivier, E., Raulet, D., Moretta, A., & Caligiuri, M. A. (2012). Innate or Adaptive Immunity? The Example of Natural Killer Cells. Science, 44-49.
Wissinger, E. (2016). CD8+ T Cells. London, UK : British Society for Immunology.
