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Understanding the interaction between HIV and developing thymocytes is crucial in determining how HIV infection perturbs the immune system. We determined which thymocyte subsets can harbor and express HIV.HIV expression in mature and immature thymocytes obtained from surgical specimens from non-infected children was determined after in vitro infection with the syncytium-inducing, cytopathic NL4–3 and the non-syncytium-inducing, relatively noncytopathic JR-CSF isolates.Intracellular staining for the HIV p24gag antigen was combined with cell surface phenotyping to determine thymocyte subsets expressing HIV. Infection was quantitated by polymerase chain reaction on sorted subsets.NL4–3 replicated faster and to higher titers and caused a more severe decrease of all CD4-bearing thymocytes than did JR-CSF. In addition, both immature CD1+ and mature CD1− thymocytes expressed NL4–3, whereas only mature CD1 -cells expressed JR-CSF. The tropism of NL4–3 for these immature cells suggests a mechanism for a more profound impact on T-cell maturation than that seen with JR-CSF. We also found that thymocytes lacking cell surface CD4 (CD4-CD8− and CD4-CD8+ subsets) expressed virus with either isolate late in infection, when viral levels were high. The CD4-CD8− cells expressing HIV were mature CD3bright T-cell receptor (TCR)α/βbright cells.These results show that NL4–3 can be expressed by thymocytes at immature and mature stages of differentiation and cause severe loss of CD4+ cells. Thus, tropism of a virus for immature cells can affect the capability of the thymus to produce new T lymphocytes leading to a greater impact on development and functions of the immune system. It is proposed that this in vitro model can be used to study pathogenic mechanisms in the thymus.