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Human T cell leukemia virus type 1 (HTLV-1) and 2 (HTLV-2) are distinct oncogenic retroviruses that infect several cell types, but display their biologic/pathogenic activity only in T lymphocytes. HTLV-1 is associated with adult T cell leukemia, a malignancy of mature CD4 + T cells, and a chronic neurological disorder termed HTLV-1-associated myelopathy/tropical spastic paraparesis (HAM/TSP). HTLV-2 is less pathogenic and has been associated with a few cases of a variant of hairy cell leukemia and neurological disease. Previous studies have indicated that in vivo HTLV-1 has a preferential tropism for CD4 + T cells, whereas HTLV-2 in vivo tropism is less clear, but appears to favor CD8 + T cells. The molecular mechanism that determines the cellular tropism of HTLV-1 and HTLV-2 has not been precisely determined. However, one study by our group has provided evidence that HTLV-1-enhanced viral transcription in CD4 + T cells may be responsible for its tropism. In an effort to understand HTLV-2 tropism we tested the ability of HTLV-2 to infect, replicate in, and transform purified CD4 + or CD8 + T cells in cell culture. After cocultures of purified primary human CD4 + and CD8 + T cells with an HTLV-2-producer cell line we measured viral transcription by reverse transcription PCR analysis, virus production by p19 gag ELISA, proviral integration by DNA slot-blot analysis, surface phenotype by FACS analysis, and cellular transformation. We also measured HTLV-2 long terminal repeat-directed transcription in the presence and absence of Tax in purified CD4 + and CD8 + T cells, using transient transfection assays. Our data indicate that CD4 + and CD8 + T cells are equally susceptible to HTLV-2 infection. We observed no significant difference in viral transcription based on mRNA and virus production in CD4 + and CD8 + T cell cocultures. Although LTR transcription was enhanced 12- to 16-fold in the presence of Tax, there was no significant difference in CD4 + or CD8 + T cells. Interestingly, we show that HTLV-2 preferentially transforms CD8 + T cells in culture. Together, our data indicate that, unlike HTLV-1, HTLV-2 cell tropism is not due to inhibition of viral infection and inefficient gene expression in CD4 + versus CD8 + T cells, and likely involves unique interactions with viral and CD8 + T cell-specific proteins.

Rearrangement of the T cell antigen receptor genes is a complex, highly regulated process. To gain a better understanding of the extracellular factors involved in the regulation of TCR beta and gamma gene rearrangement in adult murine bone marrow-resident precursor T cells, several cytokines were tested for their ability to induce gene recombination. A selected population of C58/J bone marrow cells (Thy 1(low), CD3, CD8, B220) that is enriched for pre-T cell activity was propagated in vitro in medium supplemented with IL-3 and mast cell growth factor (MGF, also referred to as stem cell factor, Steele factor and c-kit ligand). These cytokines were required for the maintenance of pre-T cell activity in culture, but had no effect on TCR gene expression. Several additional cytokines were added to the culture medium. Of all those tested, only IL-7 induced complete rearrangement of the TCR gamma locus. Complete rearrangement of the TCR beta locus was not induced under any of the culture conditions analysed here. The bone marrow cells cultured in IL-3, MGF and IL-7 did not begin to express mature T cell proteins and maintained their in vivo progenitor potential. Furthermore, IL-7 cultured bone marrow cells were capable of differentiation in vivo into all phenotypic subpopulations of T cells, without an apparent bias toward the gammadelta lineage. The data presented here suggest that TCR gamma gene rearrangement in adult pre-T cells is regulated by IL-7, but that the TCR beta locus requires additional or alternative signals for the induction of complete rearrangement.

In the adult mouse, the earliest thymocytes are derived from bone marrow-resident T lymphocyte precursor (pre-T) cells that immigrate to the thymus. There they undergo maturation through a series of developmental steps that include rearrangement and expression of the TCR genes, positive and negative selection, and functional maturation. Although these intrathymic processes have been extensively characterized, little is known about the T cell-specific events that take place in the bone marrow microenvironment. Of particular interest are the events surrounding transcription and rearrangement of the various TCR chains that are required for functional TCR expression. We have previously reported the transcription of incompletely rearranged TCR beta genes in pre-T cell-containing fractions of adult bone marrow. Here we demonstrate that the TCR gamma chain genes are also transcriptionally active in these cells. Like the TCR beta transcripts, TCR gamma transcripts are sterile, originating from unrearranged gamma loci. Interestingly, both RAG-1 and RAG-2 transcripts were also detected in this cell fraction, suggesting that sterile TCR transcription might be dependent upon the presence of a functional recombinase system. However, both C beta and C gamma sterile transcripts could be detected from the same bone marrow cell population isolated from RAG-1 gene deficient mice. Therefore, the expression of TCR genes can initiate at the earliest stages of T cell development, prior to exposure to the thymic microenvironment, and a functional recombinase system is not required for the production of these sterile TCR transcripts.