Background: CCR5 tropic HIV-1 isolates (R5 HIV-1), predominate following transmission and during the clinically latent period of infection. Moreover, R5 HIV-1 strains persist throughout infection and are the only type of HIV-1 found in approximately half of AIDS patients. The pathogenesis of R5 HIV-1 is not well understood, but involvement of the thymus is a common and detrimental component of HIV-1 infection. Therefore, we studied the replication and cytopathic effects (CPE) of several R5 HIV-1 biological clones from progressors (P) and long-term non-progressors (LTNP) in fetal thymic organ culture (FTOC).

Methods: Fetal thymic fragments were either mock infected or infected with HIV-1 biological clones and cultured at the liquid air interface for 6 to 21 days in the presence or absence of cytokines. Thymocytes were stained for CD3, CD4, CD8, CCR5 and internal p24. The percentage of CD4⁺ thymocytes and ratio of mature CD4⁺ to CD8⁺ thymocytes were used to assay the CPE of R5 HIV-1 biological clones. Culture media was collected for p24 assay to determine the viral load and for cytokine assays.

Results: We found that R5 HIV-1 clones from P but not LTNP are cytopathic in untreated FTOC. Moreover, R5 HIV-1 clones from P replicate better than LTNP derived R5 HIV-1 clones in this system. In contrast, when FTOC was maintained in the presence of IL-2, IL-4, and IL-7, both P and LTNP clones replicated similarly and induced similar CPE. IL-2 induced CCR5 expression, particularly on CD4⁺CD27⁺ thymocytes in FTOC and CCR5, expression may determine the CPE of R5 HIV-1 in FTOC, particularly for LTNP isolates. R5 HIV-1 clones showed greater pathogenesis for CCR5⁺ cells, but also showed evidence of CPE on CCR5^- cells. Furthermore, infection of FTOC by R5 HIV-1 derived from P or X4 HIV-1 induced a similar pattern of increased cytokine secretion and thymocyte MHC class-1 expression. In particular, the induction of IL-10 and TGF-b were noteworthy since these two cytokines have been implicated in CCR5 up-regulation. Induction of CCR5 expression by HIV-1 may permit thymic CPE by R5 HIV-1. These changes in thymic homeostasis may also contribute to indirect CPE and the destruction of thymic structure as a result of infection by HIV-1.

Conclusions:

1. IL-2 induces CCR5 expression on thymocytes in FTOC.
2. CCR5 expression determines CPE of R5 HIV-1 in FTOC, particularly for LTNP isolates.
3. MHC class-I and cytokine expression are induced by HIV-1 infection of FTOC.