Donatella Buonfiglio

The T cell activation molecule H4 and the CD28-like molecule ICOS are identical.

The recently cloned CD28‐like molecule ICOS displays striking similarities with H4, characterized some years ago in the mouse and recently in humans. Both molecules are selectively expressed by activated and germinal center T cells, display similar structure, and display co‐stimulatory activities. H4 displays lateral association with the CD3/TCR and is expressed by mature thymocytes. In the mouse, H4 is also expressed at high levels by thymic NKT cells that are resistant to negative selection. The aim of this work was to evaluate whether H4 and ICOS are the same molecule using the C398.4A (binding human and mouse H4) and F44 (binding human ICOS) monoclonal antibody (mAb) in parallel experiments on human T cells. ICOS and H4 displayed the same expression pattern in a panel of T cell lines and the same expression kinetics in phytohemagglutinin‐activated T cells. C398.4A completely blocked cell staining by F44, whereas F44 partially blocked C398.4A. H4 and ICOS immunoprecipitates displayed identical SDS‐PAGE patterns and H4 immunoprecipitation completely removed ICOS from cell lysates. Finally, the C398.4A mAb specifically stained cells transfected with the human or mouse ICOS. These data prove that H4 and ICOS are the same molecule and that F44 and C398.4A bind partially different epitopes.

Characterization of a novel human surface molecule selectively expressed by mature thymocytes, activated T cells and subsets of T cell lymphomas.

We have previously characterized mouse H4 (mH4), a surface glycoprotein recognized by the C398.4A monoclonal antibody. We now show that C398.4A also binds its human putative homolog (hpH4). Both hpH4 and mH4 (1) are selectively expressed by activated T cells and mature thymocytes, (2) are disulfide‐linked dimers of two chains (29/37 kDa in humans, 25/29 kDa in mice), whose N‐deglycosylation produces a single band at 20 – 21 kDa, and (3) display a low association with CD4 and the TCR. The expression pattern of hpH4 and its biochemical features showed that it is different from other known activation molecules, and this was confirmed when analysis of the tryptic digest of the hpH4 29‐kDa band by peptide mass searching using matrix‐assisted laser desorption ionization mass spectrometry did not reveal any significant homology with other molecules. In normal lymphoid tissue, hpH4 is expressed by T cells located at the periphery of lymph node germinal centers and paracortical areas. In T cell neoplasia, expression of hpH4 clusters with a subset of peripheral T cell lymphomas with a large‐cell component, and with cases of angioimmunoblastic T cell lymphomas. Overall, these data provide evidence for a novel T cell activation molecule that could help in the phenotypic categorization of T cell malignancies.

Expression of the Novel T Cell Activation Molecule hpH4 in HIV-Infected Patients: Correlation with Disease Status

We have described hpH4, a surface glycoprotein selectively expressed by activated T cells and mature thymocytes and displaying weak lateral association with CD4. The hpH4 expression pattern and biochemical features, together with analysis of its tryptic digest by peptide mass searching using MALDI-MS, suggested that it is a novel molecule. The aim of this work was to evaluate the peripheral blood T cell expression of hpH4 in HIV-infected patients and the interplay between HIV gp120 and hpH4, since both molecules interact with CD4. hpH4 expression during HIV-1 infection was evaluated by assessing 55 patients at various disease stages and following up 3 patients with primary infection and 3 patients with AIDS. hpH4 expression displayed a peak in the early phase of primary infection, dropped to control levels in the asymptomatic phase, and was newly expressed, at low levels, as AIDS developed. The expression kinetics were different than those shown by HLA-DR, CD25, and CD38. The most striking findings were the transient hpH4 expression peak displayed in the earliest stage, which was unique for hpH4. Incubation of T cells from normal donors with HIV gp120 induced transient hpH4 expression in resting CD4+ T cells and potentiated the hpH4 lateral association with CD4 in activated T cells. Moreover, hpH4 triggering inhibited gp120-induced death of CD4+ cells. Therefore, H4 expression may be a response to avoid apoptosis induced by HIV products.

The cell death-inducing ability of glycoprotein 120 from different HIV strains correlates with their ability to induce CD4 lateral association with CD95 on CD4+ T cells.

CD4 cross-linking by HIV gp120 triggers CD4+ T cell death. Several authors have suggested that this effect is mediated by CD95, but this possibility is debated by other authors. In a previous work, we found by co-capping that gp120(451) and gp120MN, but not gp120(IIIB), induce lateral association of CD4 with CD95 on the T cell surface. In this work, we used fluorescence resonance energy transfer to confirm that CD4/CD95 lateral association is induced by gp120(451), but not gp120(IIIB). Moreover, we found that gp120 ability to induce the CD4/CD95 association correlates with ability to induce cell death, since gp120(451) and gp120MN induced higher levels of cell death than did gp120(IIIB) in PHA-derived CD4+ T cell lines. CD95 involvement in gp120-induced cell death was confirmed by showing that gp120(451) and gp120MN did not induce death in CD4+ T cells derived from patients with autoimmune/lymphoproliferative disease (ALD) and decreased CD95 function. Cell death induced by gp120MN was inhibited by a recombinant CD95/IgG.Fc molecule blocking the CD95/CD95L interaction. However, inhibition was late and only partial. These data suggest that the gp120-induced CD4/CD95 association exerts a dual effect: an early effect that is independent of CD95L and may be due to direct triggering of CD95 by gp120, and a late effect that may be due to sensitization of CD95 to triggering by CD95L. In line with the former effect, cell treatment with gp120MN activated caspase 3 in the presence of Fas/IgG.Fc, which shows that cell death induced by gp120MN independently of CD95L uses the same pathway as CD95.