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thymic humoral factors (THFs) 697 thymoma
Hassall's corpuscle.
Normal thymus. This child's thymus shows the dense cortex composed
predominantly of lymphocytes and the less dense medulla with few lymphocytes.
Note the Hassall's corpuscle.
a hormone that fits the accepted classical endocrine and
physiological criteria: it is thymus-restricted and -regulated
in its secretions.
thymic humoral factors (THFs)
Soluble substances such as thymosins, thymopoietin, and
serum thymic factor that are synthesized by the thymus and
govern differentiation and function of lymphocytes.
thymic hypoplasia
An immunodeficiency that selectively affects the T cell
limb of the immune response. Early symptoms soon after
birth may stem from associated parathyroid abnormalities
leading to hypocalcemia and heart defects that may lead
to congestive heart failure. Lymphopenia is present, with
diminished T cell numbers. T lymphocyte function cannot
be detected in peripheral blood T cells. Antibody levels and
functions vary. The condition has been successfully treated
by thymic transplantation. Some patients with DiGeorge
syndrome may have normal B cell immunity. The others
may have diminished immunoglobulin levels and may not
form specific antibody following immunization. Patients
may exhibit fish-shaped mouths and abnormal faces with
low-set ears, hypertelorism, and antimongoloid eyes, in
addition to the factors mentioned above.
thymic involution
Following puberty, inhibition of double-negative (CD4 – /
CD8 – ) thymocyte proliferation leads to the progressive loss
of thymic tissue mass and function. Tissue replacement is
mainly by fat deposits.
thymic leukemia (TL) antigen
Epitope on thymocyte membrane of TL + mice. As the
T lymphocytes mature, antigen disappears, but resurfaces
if leukemia develops. TL antigens are specific and are
normally present on the surfaces of thymocytes of certain
mouse strains. They are encoded by a group of structural
genes located at the Tla locus, in the linkage group IX,
very near the D pole of the H-2 locus on chromosome 17.
There are three structural TL genes, one of which has two
alleles. The TL antigens are numbered from 1 to 4, specifying
four antigens: TL.1, TL.2, TL.3, and TL.4. TL.3
and TL.4 are mutually exclusive; their expression is under
the control of regulatory genes apparently located at the
same Tla locus. Normal mouse thymocytes belong to three
phenotypic groups: Tl–, Tl.2, and Tl.1, 2, 3. Development
of leukemia in mice induces a restructuring of the TL
surface antigens of thymocytes with expression of TL.1
and TL.2 in TL– cells, expression of TL.1 in TL.2 cells,
and expression of TL.4 in TL– and TL.2 cells. When normal
thymic cells leave the thymus, the expression of TL
antigen ceases. Thus, thymocytes are TL + (except TL–)
and the peripheral T cells undergo antigenic modulation.
In transplantation experiments, TL + tumor cells underwent
antigenic modulation. Tumor cells exposed to homologous
antibody stop expressing the antigen and thus escape lysis
when subsequently exposed to the same antibody plus
complement.
thymic medullary hyperplasia
Germinal centers present in the thymic medullae in patients
with myasthenia gravis; however, normal thymus glands
occasionally contain germinal centers, although most do not.
thymic nurse cells
Relatively large epithelial cells near thymic lymphocytes
that are believed to play a significant role in T lymphocyte
maturation and differentiation.
thymic stroma
Thymic reticular epithelial cells and connective tissue that
constitute the principal microenvironment for development
of T cells.
thymic stromal-derived lymphopoietin (TSLP)
A cytokine isolated from a murine thymic stromal cell
line that possesses a primary sequence distinct from other
known cytokines. The cDNA encodes a 140-amino acid
protein that includes a 19-amino acid signal sequence.
TSLP stimulates B220 + bone marrow cells to proliferate
and express surface μ. It synergizes with other signals to
induce thymocyte and peripheral T cell proliferation but is
not mitogenic for T cells alone.
thymin
A hormone extracted from the thymus whose activity
resembles that of thymopoietin.
thymocyte
T cell precursors that develop in the thymus beginning with
a double-negative (CD4 – /CD8 – ) stage followed by a doublepositive
(CD4 + /CD8 + ) phase and finally become single-positive
as CD4 + /CD8 – or CD4 – /CD8 + prior to release into the
periphery as mature CD4 + or CD8 + T lymphocytes.
thymoma
A rare benign neoplasm of epithelial cells often with associated
thymic lymphoproliferation. Half of these tumors
occur in patients with myasthenia gravis. They may also be
associated with immunodeficiency.
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