Blood Cells - Tissue Functioning and Remodeling in the Circulatory and Ventilatory Systems - page 125

3.10.4.3 Mucosal-Associated Invariant T Lymphocytes

Subsets of non-conventional T cells restricted by the non-polymorphic MHC

class-1-like molecules CD1d and MHC-related protein MR1 correspond to invariant
NKT cells , or CD1d-restricted natural killer T cells, and mucosa-associated invari-
ant T cells (MAIT), respectively. These lymphocytes are termed non-conventional,
innate-like, or transitional T cells, owing to some properties shared with innate
immunocytes.
Mucosal-associated invariant T cells constitute a subpopulation of T lymphocytes
that preferentially reside in the lamina propria and Peyer's patches of the gut (1-4%
of total human blood T-cell population). They share some features of invariant
NKT cells, hence their other name, mucosal NKT cells (mNKT). They target highly
conserved microbial antigens [ 232 ]. They are restricted to the monomorphic major
histocompatibility complex class-1b molecule MR1 (MHC-related molecule-1).
Mucosa-associated invariant T cells express a semi-invariant TCR rearrangement
composed of an invariant TCR

γδ

α chain (V α 7.2-J α 33 in humans), hence their third

name, MR1-restricted V α 19i T-cells. This invariant α chain associates with a

limited number of TCR 13 chains in humans). These T-cell

receptors recognize MHC-related molecule MR1. MAIT cells are activated by cells
infected with various bacteria and yeast, but not virus [ 233 ].

β chains (V β 2andV β

3.10.4.4 Memory T Lymphocytes

Development of an adaptive immune response specific for an invading pathogen
involves activation and proliferation of T lymphocytes and subsequent regulated
migration to infection sites. Effector T lymphocytes that are attracted to inflamma-
tion sites can enter any inflamed non-lymphoid tissue to prime an adequate immune
response.
Immunological memory is done either by circulating antibodies or expanded
populations of antigen-specific B and T lymphocytes with effector functions, such
as recognition of previously encountered pathogens and reactivity toward allergens
and other non-infectious antigens.
During an infection, the immune system not only produces a rapid pathogen-
specific response to destroy an invading pathogen, but also generates among effector
T cells recruited to the infection site long-lived, antigen-specific memory T cells that
can persist at peripheral sites for many months.
Memory T lymphocytes in peripheral tissues are more resistant to apoptosis than
circulating homologous cells. After meeting a known antigen, circulating antibodies
bind the corresponding antigen for elimination by phagocytic cells, whereas certain
types of immunocytes present this antigen and secrete cytokines for expansion of
antigen-specific memory B and T lymphocytes.
After the resolution of an immune response, antigen-specific memory T cells
reside in many body's organs, where they provide a first line of defense against