David Sansom

Decreased proliferative capacity and increased susceptibility to activation-induced cell death in late-passage human CD4+ TCR2+ cultured T cell clones.

The growth characteristics in vitro of interleukin 2 (IL 2)-dependent human CD4+ alpha beta-T cell receptor-positive helper T cell clones (TCC) were studied in relation to alterations in surface phenotype, cytokine responsiveness, and susceptibility to activation-induced cell death (AICD). TCC derived from peripheral blood T cells had finite lifespans averaging 33 population doublings (PD) with a recorded maximum lifespan of 80 PD (n = 208). First analyses of the TCC were undertaken at ca. 25 PD, at which time all cells of all TCC expressed high intensity CD45RO and low intensity CD45RA, as well as high intensity CD95 (fas) and MHC class II antigens. The expression of these molecules remained elevated throughout the proliferative lifespan of the clones, but for those TCC which were initially CD28+ (the majority), the density of expression of the latter was diminished in most late-passage clones. Concomitant with this, late-passage cells showed reduced responsiveness to CD28-mediated costimulation by CHO transfectants expressing human CD80 compared to early-passage cells. Additionally, the level of expression of IL 2R gamma c and IL 7R chains was commonly reduced, as was the response to IL 2 and IL 7. Despite unchanged levels of fas expression on TCC with time, late-passage cells were more susceptible to AICD than early, passage cells. These observations further document functional and phenotypic alterations in long-term cultured human T helper cells, which may be considered as biomarkers of immunosenescence. This may contribute to an improved understanding of the mechanisms underlying depressed T cell function in old age.

The Role of Endogenous Peptides in the Direct Pathway of Alloreactivity to Human MHC Class II Molecules Expressed on CHO Cells

Allograft rejection is caused by the recognition of major histocompatibility complex {MHC) molecules by recipient T cells via two different pathways. The indirect pathway of alloreactivity requires the presentation of MHC peptides derived from the graft by the autologous antigen-presenting cells of the host. The direct pathway, on the other hand, requires the recognition of foreign MHC molecules on graft cells, apparently in an MHC-unrestricted fashion. It is believed that antigen receptors on T cells recognize foreign MHC molecules because of molecular mimicry (i.e. foreign MHC determinants and self MHC/foreign peptide complexes are recognized by virtue of cross-reactivity), and that the endogenous peptides bound by the MHC contribute to the generation of the epitope recognized {Lechler et al. 1990). A great deal of circumstantial evidence in favour of an important role for peplides in alloreactivity has been presented, ranging from the tissue specificity of ailorecognition to the finding that stimulation by MHCpositive antigen-processing mutants is impaired {Sayegh et al. 1994). However, despite the general acceptance of this hypothesis for class I-directed alloreactivity (Heath et al. 1989, Rotzschke et al. 1991), there are very few published examples