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Antibody regulation of T-cell immunity: implications for vaccine strategies against intracellular pathogens

Intracellular microbial pathogens cause a plethora of diseases that pose a huge public health challenge. Efficacious prophylactic vaccines are needed to protect the population from this myriad of infectious diseases. Contemporary approaches to vaccine design are guided by the immunobiological paradigm that extracellular pathogens are controlled principally by humoral immunity, involving specific antibodies, whereas host protection against intracellular pathogens requires effectors of cell-mediated immunity. However, this distinct T-helper (Th) type 1 and 2 paradigm of host defense has encountered a major challenge due to the reality that most antigens or vaccines induce mixed immune responses comprising of both humoral and CMI effectors. Besides, the true functional independence of antibodies and T-cells under in vivo physiologic conditions is uncertain. Recent findings have revealed that antibodies exert a significant immunoregulatory effect on T-cell immunity. Thus, a robust and protective T-cell memory response against microbial pathogens such as Chlamydia and Mycobacteria require an effective primary humoral immune response characterized by specific antibody isotypes whose role is to modulate Th1 activation via Fc receptors (FcR) by facilitating a rapid uptake, processing and presentation of pathogen-derived antigens for an enhanced T-cell response. These findings have crystallized into a paradigm shift in host defense wherein different components of the apparently disparate mixed immune responses elicited against a microbial pathogen function concertedly to maximize the principal effector mechanism. This review focuses on the essential role of both arms of the immune system in controlling intracellular microbial pathogens, especially the regulatory role of FcR-mediated antibody function in optimizing the induction of a protective Th1 response. The immunobiological implications are discussed in the context of vaccine design, delivery and evaluation against intracellular microbial pathogens of bacteria, fungi and parasitic origin.