Page 209 - AIDSBK23C

Page 209

THYMUS IN AIDS

The thymus may play a role in evolution of immunologic consequences of HIV infection.
Though the thymic production of naïve CD4 and CD8 lymphocytes decreases with age,
functional thymic tissue is still present in adults. The CD4 lymphocytes in the thymus are
preferentially targeted by CXCR4-tropic strains of HIV. HIV-1-induced immune activation
leads to a higher rate of differentiation of naïve lymphocytes into the effector/memory pool. An
increase in thymic activity seems to be part of the process that maintains peripheral CD4 cells
during the latent, asymptomatic phase of HIV infection. Thymic activity may play a role in the
immune reconstitution that occurs during HIV antiretroviral treatment.[859]
The thymus is not a significant site of pathologic lesions in adult AIDS patients. Thymic
tissue in adults is not grossly prominent or microscopically cellular under normal circumstances,
and is no different in patients with AIDS. Opportunistic infections are rare. Even malignant
lymphomas, typically widespread in AIDS, do not involve the thymus.[860]
In adults infected with HIV-1, a B-lymphocytic follicular hyperplasia can be identified in
thymic lymphoid tissue. The germinal centers are infiltrated by plasma cells. This hyperplasia is
similar to that found in lymph nodes in the same HIV-infected person. Small numbers of
lymphocytes can be shown to contain HIV-1 RNA, consistent with the role of the lymphoid
tissue as a reservoir for HIV during the latent stage of infection.[861]
In pediatric AIDS, specific thymic pathology has been observed to consist of precocious
involution, involution mimicking thymic dysplasia of congenital immune deficiency and/or
thymitis. HIV may produce the lesions by injury to thymic epithelial cells. Severe, early thymic
injury may be irreversible and further diminish cell-mediated immunity in infected
children.[862]
Findings in the thymus at autopsy in pediatric patients with HIV infection, most of whom
died from AIDS, may include severe lymphoid depletion with atrophy, microcystic
transformation of Hassall’s corpuscles, calcification of Hassall’s corpuscles, plasma cell
infiltrates, and Warthin-Finkeldey type giant cells.
Some pediatric patients may develop multilocular thymic cysts. These lesions are
typically discovered incidentally by a routine chest radiograph that demonstrate an anterior
mediastinal mass. The children have no symptoms related to these masses. By computed
tomographic scan, the mass can appear multicystic. Histologic findings include distortion of
thymic architecture with focal cystic changes, follicular hyperplasia, diffuse plasmacytosis, and
the presence of multinucleated giant cells. The irregular cystic spaces are lined by a keratin
positive flattened epithelium. No malignant changes occur. The presence of Epstein-Barr virus
can be demonstrated in lymphoid tissue in some cases. In over half of cases, the mass decreases
in size or resolves completely over time.[863]