Blood Cells - Tissue Functioning and Remodeling in the Circulatory and Ventilatory Systems - page 127

peripheral tissues. Effector and central memory T lymphocytes have distinct effector
functions in response to antigen stimulation. Associated with dendritic cells, they
both produce large amounts of interferon- and tumor-necrosis factor. However,

T CM cells synthesizes more interleukin-2 and much less lytic granzyme-B and
perforin than T EM cells. In addition, a small T EM fraction expresses low-affinity
IgG Fc

γ

R3a receptor.
Vaccines against chronic infections and tumors aim at inducing effective memory
T-cell responses, especially memory CD8

γ

+ T cells. Target of rapamycin is a major

regulator of memory CD8 T-cell differentiation [ 236 ]. Rapamycin increases the

number of memory precursors during the expansion phase of the T-cell response.
During the contraction phase of the T-cell response that corresponds to a transition
from effector T cells to memory T cells, rapamycin accelerates the memory T-
cell differentiation program. Kinase TOR acts via the TOR complex-1 to regulate
memory T-cell differentiation.

+

Distribution of Memory T Lymphocytes

Following the clearance of an invading pathogen, memory T lymphocytes arise
in both lymphoid and peripheral exposed organs (mostly skin and digestive,
respiratory, and genital tract) that are major portals of pathogen entry into the body.
Although memory T lymphocytes are widely disseminated throughout the body, the
distribution of pathogen-specific memory T cells depends on the type of initially
infected tissue. Sites of restricted entry, such as the brain, lung airways, intestine
lamina propria, and dermis, are characterized by a greater number of memory
T lymphocytes, whereas numbers of memory T cells in sites of common entry,
such as lung parenchyma, lymph nodes, spleen, liver, and bone marrow, are often
similar [ 235 ].

Migration of Memory T Lymphocytes

Migration of circulating memory T lymphocytes into non-lymphoid peripheral
tissues for effective surveillance is more restricted than that of common effector
T cells. Memory T cell migration into peripheral tissues is controlled by combi-
nations of adhesion molecules and chemokine receptors expressed by T cells and
complementary adhesion molecules and chemokines expressed by cells in periph-
eral sites (Table 3.28 ). Effector T lymphocyte expression of adhesion molecules and
chemokine receptors is determined during initial antigen contact by molecular cues
governed by local environment [ 235 ]. Antigen-presenting cells such as dendritic
cells from lymph nodes together with local non-hematopoietic peripheral cells and
local draining lymph node-resident stromal cells are necessary and sufficient to
program T cells with a tissue-tropic phenotype.