CD45RA antibodies split the CD3bright T cell subset

Interleukin-7 promotes the generation of phenotypically mature CD45RA positive human thymocytes in vitro

IL-7 has been shown to support the proliferation of several cell types including human and murine thymocytes. We have investigated the influence of IL-7 on human thymocyte growth and maturation. In a serum-free culture system, recombinant human interleukin-7 (IL-7) (200 U/ml) stimulated nylon wool purified human thymocytes to express a mature phenotype. IL-7 promoted the generation of a population of large thymocytes which expressed high density CD3, CD4 and/or CD8, and high density CD45RA after 10 to 14 days in culture. The induction of CD45RA was not a transient event, since large CD45RA+ cells remained "bright" CD45RA+ up to 12 days after IL-7 was removed from cultures. IL-7 did support thymocyte proliferation. However, the IL-7-induced generation of large CD45RA+ thymocytes was not due to proliferation of a small subset because large CD45RA+ thymocytes also appeared in the presence of proliferation inhibitors. In order to identify IL-7-responsive thymocyte subsets, highly purified CD45RA- subsets at distinct stages of maturation were tested. CD3-, "dim" CD3+ and "bright" CD3+ subsets each responded to IL-7 with de novo expression of CD45RA. These data provide evidence that IL-7 supports phenotypic changes in both mature and immature subsets of human thymocytes, and indicate that IL-7 may play an important role in T cell development in the thymus.

CD45RA and CD45RBhigh expression induced by thymic selection events

CD45 is a protein tyrosine phosphatase involved in T and B cell signaling. While peripheral T cells switch CD45 isoforms upon activation, events leading to exon switching during T cell development in the thymus have not been determined. The expression of high molecular weight isoforms of CD45 was examined on thymocytes from nontransgenic and T cell receptor (TCR) transgenic mice. All thymocytes from nontransgenic mice were CD45RB+ as assessed by staining with MB23G2, an anti-CD45RB-specific monoclonal antibody. Interestingly, there was a small population (1-3%) of thymocytes that displayed a higher intensity of staining with MB23G2, CD45RBhigh. CD45RBhigh thymocytes were found in all subsets defined by CD4 and CD8 expression and were also present within the TCR-alpha/beta high population. To analyze whether or not CD45 expression correlated with thymic selection events, expression of CD45RBhigh and a second isoform, CD45RA, was examined on thymocytes from H-Y and 2C TCR transgenic mice and found to correlate with positive and negative selection events but did not occur in nonselecting backgrounds. CD45RA and CD45RBhigh upregulation was also not observed in transgenic mice backcrossed into CD8-deficient mice, a scenario in which there is no positive selection of transgene-expressing thymocytes. These data suggest that modulation of CD45 isoform expression may be involved in thymic selection events.

The development of functionally responsive T cells

The work reviewed in this article separates T cell development into four phases. First is an expansion phase prior to TCR rearrangement, which appears to be correlated with programming of at least some response genes for inducibility. This phase can occur to some extent outside of the thymus. However, the profound T cell deficit of nude mice indicates that the thymus is by far the most potent site for inducing the expansion per se, even if other sites can induce some response acquisition. Second is a controlled phase of TCR gene rearrangement. The details of the regulatory mechanism that selects particular loci for rearrangement are still not known. It seems that the rearrangement of the TCR gamma loci in the gamma delta lineage may not always take place at a developmental stage strictly equivalent to the rearrangement of TCR beta in the alpha beta lineage, and it is not clear just how early the two lineages diverge. In the TCR alpha beta lineage, however, the final gene rearrangement events are accompanied by rapid proliferation and an interruption in cellular response gene inducibility. The loss of conventional responsiveness is probably caused by alterations at the level of signaling, and may be a manifestation of the physiological state that is a precondition for selection. Third is the complex process of selection. Whereas peripheral T cells can undergo forms of positive selection (by antigen-driven clonal expansion) and negative selection (by abortive stimulation leading to anergy or death), neither is exactly the same phenomenon that occurs in the thymic cortex. Negative selection in the cortex appears to be a suicidal inversion of antigen responsiveness: instead of turning on IL-2 expression, the activated cell destroys its own chromatin. The genes that need to be induced for this response are not yet identified, but it is unquestionably a form of activation. It is interesting that in humans and rats, cortical thymocytes undergoing negative selection can still induce IL-2R alpha expression and even be rescued in vitro, if exogenous IL-2 is provided. Perhaps murine thymocytes are denied this form of rescue because they shut off IL-2R beta chain expression at an earlier stage or because they may be uncommonly Bcl-2 deficient (cf. Sentman et al., 1991; Strasser et al., 1991). Even so, medullary thymocytes remain at least partially susceptible to negative selection even as they continue to mature.(ABSTRACT TRUNCATED AT 400 WORDS)