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tissue transglutaminase autoantibodies 705 Tla complex
Arne W.K. Tiselius.
expressed inappropriately in a neoplastic cell, it may be
called a tumor-associated antigen.
tissue transglutaminase autoantibodies
Immunoglobulin A (IgA) autoantibodies against tissue
transglutaminase (tTG) that have a close correlation with
active celiac disease. tTG is an intracellular enzyme found
in the endomysium and is released after cellular injury or
wounding. The enzyme facilitates deamidation of extracellular
proteins including gluten peptides, leading to
enhanced T cell stimulatory activity. tTG antibodies are
transient, disappearing when gluten is eliminated from the
diet and mucosal healing occurs. Available data suggest that
tTG autoantibodies are very specific for celiac disease.
tissue typing
The identification of major histocompatibility complex
(MHC) class I and class II antigens on lymphocytes by
serological and cellular techniques. The principal serological
assay is microlymphocytotoxicity using microtiter
plates containing predispensed antibodies against human
leukocyte antigen (HLA) specificities to which lymphocytes
of unknown specificity plus rabbit complement and vital
dye are added. Following incubation, the wells are scored
according to the relative proportion of cells killed. This
method is employed for organ transplants such as renal
allotransplants. For bone marrow transplants, the earlier
mixed lymphocyte culture (MLC, also called mixed lymphocyte
reaction) procedure was performed to determine
the relative degree of histocompatibility or histoincompatibility
between donor and recipient. The older cellular
and serological techniques have been replaced largely by
molecular (DNA) typing procedures employing polymerase
chain reaction (PCR) methodology and DNA or oligonucleotide
probes, especially for MHC class II typing. Sequencespecific
primer (SSP) technology is the current choice for
molecular typing. In addition to PCR-based probe hybridization
techniques, restriction fragment length polymorphisms
and direct amplicon analysis methods are also
employed. Panel-reactive antibody analysis by Luminexx ®
or an equivalent instrument is useful also for antibody identification
in post-transplant monitoring.
titer
An approximation of the antibody activity in each unit
volume of a serum sample. The term is used in serology.
Titer is determined by preparing serial dilutions of antibody
to which a constant amount of antigen is added. The end
point is the highest dilution of antiserum in which a visible
reaction with antigen (e.g., agglutination) can be detected.
Titer is expressed as the reciprocal of the serum dilution
that defines the end point. If agglutination occurs in a tube
containing a 1:240 dilution, the antibody titer is said to be
240, that is, the serum contains approximately 240 units
of antibody per milliliter of antiserum. Titer provides only
an estimate of antibody activity. For absolute amounts of
antibody, quantitative precipitation or other methods must
be employed.
TL (thymic leukemia) antigen
An epitope on the thymocyte membranes of TL + mice. As
the T lymphocytes mature, this antigen disappears, but it
resurfaces if leukemia develops. TL antigens are specific
and normally present on 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.
One of the three structural TL genes 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 both TL and TL.2 cells. When normal thymic
cells leave the thymus, the expression of TL antigen ceases.
Thus, thymocytes are TL (except the TL strains) and the
peripheral T cells are TL. In transplantation experiments,
TL tumor cells undergo antigenic modulation. Tumor cells
exposed to the homologous antibody stop expressing the
antigen and thus escape lysis when subsequently exposed to
the same antibody plus complement.
Tla antigen
Murine major histocompatibility complex (MHC) class I
histocompatibility antigen encoded by genes situated near
the Qa region on chromosome 17. Thymocytes may express
products of up to six alleles. Leukemia cells may aberrantly
express Tla antigens.
Tla complex
Genes that map to the major histocompatibility complex
(MHC) region telomeric to H2 loci on chromosome 17 in
mice. These genes encode MHC class I proteins such as Qa
and Tla that have no known immune function. Qa and Tla
proteins that closely resemble H2 MHC class I proteins in
sequence associate noncovalently with β 2 microglobulin.
Expression of Qa and Tla proteins, unlike expression of
MHC H-2 class I proteins, is limited to only selected mouse
cells. For example, only hepatocytes express Q10 protein,
only selected lymphocyte subpopulations such as activated
T lymphocytes express Qa-2 proteins, and T lymphocytes
express Tla proteins. Thus, Qa and Tla class I molecules
differ in structure and expression from the remaining
MHC class I genes and proteins in mice. This may account
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