Page 1196 - Veterinary Immunology, 10th Edition

more frequently than normal in the intestine of patients with active
VetBooks.ir ankylosing spondylitis and uveitis, and patients with active disease

have elevated levels of IgA against Klebsiella in their sera. Cloning
of B27 into mice and subsequent infection of these animals with K.

pneumoniae causes an acute spondylitis. HLA-B27–associated
ankylosing spondylitis has been described in gorillas. Up to 20% of
wild gorillas may have spondylitis, and the disease has also been
described in a gibbon, in baboons, and in rhesus macaques.

In porcine enzootic pneumonia caused by Mycoplasma
hyopneumoniae, antibodies to the mycoplasma cross-react with pig
lungs, and in contagious bovine pleuropneumonia, there is cross-
reactivity between Mycoplasma mycoides antigens and normal bovine

lung. It is not known to what extent these autoantibodies contribute
to the pathogenesis of these diseases. There is a clearer causal
relationship between Leptospira interrogans infection and the
development of periodic ophthalmia, the leading cause of blindness

in horses (Chapter 37).
Some microbial superantigens may also trigger autoimmunity.
The superantigen staphylococcal enterotoxin B activates the same T
cells that react with myelin and induces an autoimmune

encephalitis. It has been suggested that a bacterial superantigen
may trigger rheumatoid arthritis since the T cells in affected joints
are enriched in cells bearing specific TCR V domains. The only
known agents that can alter V region gene expression in this way

are superantigens.

Epitope Spreading

In some cases, autoimmunity seems to result from a normal
immune response against a foreign antigen that subsequently

“spreads” to recognize self-antigens. When an immune response is
initiated, the immune response is first directed against a single
epitope on the inciting antigen. However, as the process continues,
T and B cell receptors diversify, and responses begin to be directed
against additional epitopes. At first they will react with other

epitopes on the same protein. Eventually responses may spread to
epitopes on autoantigens. Epitope spreading has been
demonstrated in diseases such as thyrotoxicosis and diabetes

mellitus and may account for some of the difficulties encountered

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