Blood Cells - Tissue Functioning and Remodeling in the Circulatory and Ventilatory Systems - page 114

3.10.2.1 Antigen-Presenting Cells

T lymphocytes encounter their cognate antigens in specialized compartments of
secondary lymphoid organs, where antigen-presenting cells, such as dendritic cells,
B lymphocytes, basophils, and stromal cells present self and non-self antigens to
T cells. Dendritic cell subpopulations can be characterized by distinct capacity for
antigen capture, processing, and presentation.
In the thymus, epithelial cells that are specialized for immunotolerance produce
a large number of peripheral tissue-restricted antigens, mainly under the control of
the autoimmune transcriptional regulator (AIRe).
Lymph node stromal cells present tissue-restricted antigens to naive T cells that
become self-reactive T lymphocytes for immune tolerance. Fibroblastic reticular
cells, lymphatic and blood vessel endothelial cells as well as double-negative
(podoplanin

cells possess
self-antigens. 103 Fibroblastic reticular cells and lymphatic and blood endothelial
cells synthesize the transcriptional regulator Deformed epidermal autoregulatory
factor DEAF1, 104 as the transcriptional regulator AIRe does not control the ex-
pression of peripheral tissue-restricted antigens by these cells [ 220 ]. Hematopoietic
antigen-presenting cells either permanently reside in secondary lymphoid organs,
such as dendritic and stromal cells, or migrate from blood (B lymphocytes and
basophils) or lymph (dendritic cells) [ 220 ].
Lymph node-resident dendritic cells acquire lymph-borne antigens either directly
from subcapsular and medullary sinuses or indirectly via migrating dendritic cells
in perfusing and draining lymphatics [ 220 ]. Lymph nodes filter the lymph in search
of microorganisms, cellular and tissular debris, and apoptotic cells. Once in lymph
nodes, lymph is controlled by macrophages that line the subcapsular and medullary
sinuses and phagocytose most lymph-borne antigens. Most antigens are degraded by
lysosomal proteolysis, but a small amount of antigen can be transferred to B- and
follicular dendritic cells. B-cell follicles localize to the outer cortex of lymph nodes.
Follicular dendritic cells organize B-cell follicles. T lymphocytes lodge mainly in
the the inner cortex (or paracortex).
The spleen is responsible for the immunosurveillance of blood. The spleen
is composed of 2 compartments: (1) the white pulp that contains lymphocytes
and (2) red pulp, where blood is filtered through venous sinuses past cords of
phagocytes (Vol. 5 - Chap. 5. Lymphatic System). The marginal zone that separates
the white and red pulp is an antigen-capture locus, where antigens in the marginal

− and PECAM1 − ) stromal cells and extrathymic AIRe +

103 Fibroblastic reticular cells lodge in T-cell zones in the paracortex of lymph nodes. They produce
various extracellular matrix proteins that form sieve-like fibers. They ensheath these fibers to
form a conduit network [ 220 ]. They synthesize chemokines CCL19 and CCL21 that recruit
chemokine receptor CCR7

T- and dendritic cells. In addition, they secrete interleukin-7 to foster
T-cell survival. Resident dendritic cells extend protrusions into the fibroblastic reticular cell-lined
conduits to capture soluble antigens delivered by the lymph.
104 A.k.a. suppressin and zinc finger MYND domain-containing protein-5 (ZMYND5).

+