Role of cytokines in the innate immune response to intracellular pathogens

Intracellular pathogens live inside host cells, and survival is dependent on coexistence with the host. In their intracellular niche these pathogens are well shielded from the effector cells of the cellular immune system. However, microbial proteins are processed and presented, thus promoting activation of T lymphocytes. These T lymphocytes determine resistance, susceptibility, and often immunopathogenesis of intracellular infections. Pathogenic intracellular bacteria and parasites include different species of Listeria , Mycobacteria , Salmonella , Chlamydia , Rickettsia , Trypanosoma , and Leishmania . Although CD4+ lymphocytes are central to acquired resistance, an increasing amount of evidence is emerging showing crucial contributions by CD8+ T cells as well as unconventional T cells. These include CD4−, CD8− TCRαβ+ T cells recognising mycobacterial lipid antigens, T cells expressing the TCRγδ or natural killer T cells, which are thought to have a regulatory function in the early immune response. Facultative intracellular microbes favour mononuclear phagocytes as their biotope, but can survive in the extracellular environment. In contrast, obligate intracellular bacteria such as Rickettsiae and Chlamydiae must enter host cells, because their metabolism requires nutrients of the eukaryotic cell. The different types of immune response fall into two categories: innate immune response and adaptive immune responses (table1). View this table: Table 1 Different types of immune response The major difference between innate and acquired immune responses is that the latter are highly specific for a particular pathogen. Moreover, although the innate immune response does not alter on repeated exposure to a given infectious agent, the adaptive response improves with each successive encounter with the same pathogen. Because innate immunity functions at times before adaptive immunity, its major role is likely to be to initiate defence early during primary infections. There is growing appreciation of the immunoregulatory role of the innate immune responses both in activating cellular constituents of innate …