IL-7 transgenic mice: Analysis of the role of IL-7 in the differentiation of thymocytes in vivo and in vitro

We have generated a high copy number transgenic mouse line in which expression of mouse IL-7 cDNA is under the control of the mouse MHC class II E_α promoter. These mice were generated in order to see if IL-7 over-production in the thymus altered either thymocyte differentiation or the process of negative selection. Using in situ hybridization, IL-7 transcripts could be detected in the thymic cortex and medulla as well as the spleen and lymph nodes of transgenic mice but was undetectable in normal controls. Phenotypic and molecular analysis of thymocytes from embryonic and adult transgenic mice failed to reveal a dramatic effect of IL-7 on thymocyte differentiation and negative selection of the TCR V_β repertoire appeared to be intact. In peripheral lymph nodes, there was a massive (30-fold) increase in the number of T cells (CD8⁺ ≫ CD4⁺) and simultaneous presence of immature (B220⁺, Ig^-) B cells. TCR repertoire analysis showed that the expansion of peripheral T cells was polyclonal. Using the polymerase chain reaction (PCR), transgene-specific IL-7 transcripts could be detected in the thymus from day 14 of fetal development However, using semi-quantitative PCR, there was no dramatic increase in the degree of TCRβ or TCRα gene rearrangements during thymocyte ontogeny in vivo. Similarly, when fetal mouse thymus lobes were cultured with IL-7 in vitro, there was no dramatic increase in the degree of TCRβ or TCRα gene rearrangements. We conclude that IL-7 is probably not an important differentiation factor for immature mouse thymocytes.