Evidence for Susceptibility of Intrathymic T-Cell Precursors and their Progeny Carrying T-Cell Antigen Receptor Phenotypes TCRαβ+and TCRγδ+to Human Immunodeficiency Virus Infection: A Mechanism for CD4+(T4) Lymphocyte Depletion

Individuals infected by the human immunodeficiency virus type 1 (HIV-1) demonstrate progressive depletion and qualitative dysfunction of the helper T4(CD4+) cell population. Mechanisms proposed for attrition of CD4+T cells include direct cytopathicity of these mature cells following infection as well as infection of early T-lymphocyte progenitors. The latter mechanism could lead to failure to regenerate mature functioning CD4+T cells. The present study determines the susceptibility of thymocytes at various stages of maturity to infection with HIV-1. Various normal thymocyte populations were inoculated with HIV-1, including unfractionated (UF), CD3-CD4-CD8-["triple negative" (TN)], CD4+CD8+["double positive"(DP)] thymocytes, and thymocyte populations obtained by limited dilution cloning. Cultures were studied for the presence of HIV-1 DNA by polymerase chain reaction in addition to examination for reverse transcriptase activity. We determined that transformed T-cell and thymocyte cell lines completely lacking CD4 were not susceptible to infection by HIV-1, whereas all of the following lines were: UF thymocytes (70-90% CD4hi+); DP thymocytes (99% CD4hi+); TN thymocytes (0% CD4hi+); and TCRα β+, TCRγδ+, or CD16+CD3-(natural killer) thymocyte clones expressing variable levels of CD4 and representing the progeny of TN thymocytes. [TCRαβ+and TCRγδ+refer to the chains of the T-cell antigen receptor (TCR), and CD4hi refers to a strong rightward shift (>30 linear channels) of the CD4 curve on flow cytometric analysis compared with control.] Monoclonal antibodies (mAbs) to CD4 (T4a epitope) but not to CD3 (T3) were capable of blocking infection of mature and immature CD4hi+thymocytes. Moreover, anti-CD4(T4a) mAbs also inhibited infection of CD4hi-TN thymocytes, indicating that these T-cell precursors-despite their apparent "triple negativity" (CD3-CD4hi-CD8-)- expressed sufficient CD4 molecules to become infected. Cell sorted analysis with a panel of CD4mAbs demonstrated a mean shift of the mean fluorescence channel (MFC) with CD4 mAbs on TN thymocytes of 6 m 4 MFC units. Thus, intrathymic T-cell precursors and their progeny representing many stages of T-cell ontogeny are susceptible to infection by HIV-1, including early TN thymocytes, which express very low levels of CD4. Infection of multiple stages and multiple subsets of the T-cell lineage in man, mediated via the CD4 molecule, may explain the inability of the T-cell pool to regenerate in the setting of progres