Mast cell interactions with B and T cells
Mast cells communicate with many other cells of various types in the body. The type of communication we have discussed here most often is via mediator release – mast cells release mediators and they trigger an action in another cell by binding to a receptor, or the other cells release mediators that act on mast cell receptors. Another method of interaction is for cells to physically contact with each other. Mast cells use these techniques to impact the behavior of other cells.
B cells are lymphocytes, a kind of white blood cell. They form part of the adaptive immune system, the arm of immunity that is learned over the course of your life. They make antibodies when exposed to allergens or antigens from infectious agents. They help amplify the immune response during infection, can release cytokines and some become memory B cells, which allow for rapid response to a previously encountered organism.
Mast cells produce and release IL-4, IL-5, IL-6 and IL-13, all of which regulate the development of B cells and which role they develop toward. Mast cells can induce IgE production by B cells via binding of OX40 on the B cell to the OX40 receptor on the mast cell. In the absence of an activating signal, mast cells are able to cause unactivated B cells to proliferate and become IgM producing cells.
Resting and activated mast cells inhibit B cell death and promote proliferation of undifferentiated B cells. When the B cells are activated, this effect is exaggerated. These changes occur when mast cells and B cells are in contact and mast cells have released IL-6. Activated mast cells can drive B cells toward becoming CD138+ plasma cells or producing IgA.
T cells are also lymphocytes. There are several types of T cells and all perform very specialized functions. Mast cells and T cells are often found in close physical proximity in inflamed spaces. Conditions in which this commonly occurs include sarcoidosis, irritable bowel disease, rheumatoid arthritis and prolonged allergic processes.
Contact between mast cells and T cells initiates gene expression in mast cells. When the T cells are activated, it also induces mast degranulation, production/release of TNFa, release of MMP-9, inhibition of MMP-1, and release of IL-4 and IL-6. This occurs due to binding between the surface molecules LFA-1 and ICAM-1. Another receptor on mast cell surfaces, LTβR, can be bound by T cells. This initiates release of IL-4, IL-6, TNFa, CXCL2 and CCL5 by mast cells. In the presence of TNFa, binding to OX40 on activated CD4+ T cells by mast cells causes T cell proliferation and cytokine production.
Mast cells can express proteins on their surfaces called major histocompatibility complex I and II. These proteins literally show pieces of a phagocytosed, or “eaten”, pathogen. Showing these pieces to other cells allows them to fight infection in a specialized way. When mast cells express MHC II, they can steer T cells toward developing into specific types, including Treg cells. When mast cells express MHC I, they increase CD8+ T cell populations and ability to kill infectious agents. CD8+ T cells can cause MHC I expression by mast cells.
Mast cells and Treg cells are found in close proximity in secondary lymphoid and mucosal tissues. Activated Treg cells reduce the amount of IgE receptors on mast cells when they come into contact. They also cause release of TGF-b and IL-10. Treg cells interfere with degranulation via the OX40 receptor on mast cells.
Gri, Giorgia, et al. Mast cell: an emerging partner in immune interaction. Front. Immunol., 25 May 2012.
Brill, A., Baram, D., Sela, U., Salamon, P., Mekori, Y. A., and Hershkoviz, R. Induction of mast cell interactions with blood vessel wall components by direct contact with intact T cells or T cell membranes in vitro. Clin. Exp. Allergy 2004; 34, 1725–1731.