title: The Role of the Tyrosine Phosphatase CD45 in B and T Cell Development
Creators: name: Mee, Patrick Joseph
Publisher: name: The Open University
Year: 1999

description: For many years the CD45 transmembrane tyrosine phosphatase has been known to be an important molecule in both B and T lymphocyte receptor signalling events. To understand the role of this molecule in lymphocyte ontogeny and function I have created mice which are homozygous for a targeted mutation of exon 12 of the <i>Cd45</i> gene. These mice have B cells that are unusual in that they appear immature on the basis of their expression of IgM and IgD receptors and yet have increased expression of other markers that are associated with increasing B cell maturation. In addition, CD45-deficient B cells have increased expression of B7.2, suggesting that these cells have been activated. Following BCR stimulation <i>in vitro</i> CD45-deficient B cells have a diminished proliferation response. B-1 cell populations have been found to be greatly reduced in these mice suggesting that CD45 is also a positive regulator of B-1 receptor signalling. The use of the 3-83 transgenic BCR demonstrates that 383/<i>Cd45</i><sup>-/-</sup> B cells can be produced however these cells express altered levels of IgM and IgD. <i>Cd45</i><sup>-/-</sup> B cells are deleted normally in the presence of the negatively selecting ligand for the transgenic receptor and undergo both Ig heavy and light chain rearrangements normally suggesting that CD45 is not required for these processes. Finally <i>Cd45</i><sup>-/-</sup> B cells have a diminished but not absent Ca<sup>2+</sup> flux on receptor stimulation that could explain the abolition of some receptor mediated responses. <br><br> <i>Cd45</i><sup>-/-</sup> mice have a severe deficiency in T cell development. Firstly there is a developmental block at the DN3 stage of thymocyte development, suggesting that CD45 plays a role in signalling from the pre-TCR. A second developmental block is at the DP stage in thymocyte developmental which is associated with the positive and negative selection events of T cell selection. Some <i>Cd45</i><sup>-/-</sup> T cells are able to migrate to the periphery albeit in greatly reduced numbers. These cells are unusual in that they have an activated phenotype (CD44<sup>hi</sup>/L-selectin<sup>lo</sup>). To determine the role of CD45 during positive selection of thymocytes both class 1 and class II transgenic TCR receptors were used. <i>Cd45</i><sup>-/-</sup> mice were found to display a profound block in the development of mature T cells expressing the transgenic TCRs suggesting an important role for CD45 in the positive selection of these receptors. Moreover, this deficiency appears to be due to altered thresholds of TCR signalling as assessed using bi-specific antibodies to induce positive selection events in a dose dependent manner. The use of a transgenic TCR to investigate T cell negative selection events demonstrates that <i>Cd45</i><sup>-/-</sup> thymocytes can undergo negative selection events induced by the cognate ligand. However, negative selection induced by endogenous (<i>Mtv</i>) and exogenous (SEB) superantigens are deficient in <i>Cd45</i><sup>-/-</sup> thymocytes. In addition <i>Cd45</i><sup>-/-</sup> T cells are found to have a profound block in their ability to mobilise Ca<sup>2+</sup> in response to antigen receptor stimulation.