Relevance of lymphocyte proliferation to PHA in severe combined immunodeficiency (SCID) and T cell lymphopenia

Severe combined immunodeficiency (SCID) is characterized by a severe deficiency in T cell numbers. We analyzed data collected (n = 307) for PHA-based T cell proliferation from the PIDTC SCID protocol 6901, using either a radioactive or flow cytometry method. In comparing the two groups, a smaller number of the patients tested by flow cytometry had <10% of the lower limit of normal proliferation as compared to the radioactive method (p = 0.02). Further, in patients with CD3+ T cell counts between 51 and 300 cells/μL, there was a higher proliferative response with the PHA flow assay compared to the 3H-T assay (p < 0.0001), suggesting that the method of analysis influences the resolution and interpretation of PHA results. Importantly, we observed many SCID patients with profound T cell lymphopenia having normal T cell proliferation when assessed by flow cytometry. We recommend this test be considered only as supportive in the diagnosis of typical SCID.

Measurement of Calcineurin Phosphatase Activity in Cell Extracts

The calmodulin-dependent phosphatase calcineurin is an important molecular target of the immunosuppressive drugs cyclosporin A (CsA) and FK506. Complexes of CsA and FK506 with their immunophilin ligands interact with calcineurin in vitro, resulting in the inhibition of its serine/threonine phosphatase activity toward polypeptide substrates. In order to demonstrate that CsA and FK506 inhibit calcineurin in vivo, we developed an assay to measure calcineurin phosphatase activity in crude cell extracts. We have previously reported that incubation of intact cells with nanomolar concentrations of either CsA or FK506 results in potent inhibition of calcineurin activity in a variety of cell lines. Here we discuss in detail the methodology and applications of the cell extract calcineurin assay.

Characterization of a mutant calcineurin A alpha gene expressed by EL4 lymphoma cells

The calmodulin-stimulated phosphatase calcineurin plays a critical role in calcium-dependent T-lymphocyte activation pathways. Here, we report the identification of a missense mutation in the calcineurin A alpha gene expressed by EL4 T-lymphoma cells. This mutation changes an evolutionarily conserved aspartic acid to asparagine within the autoinhibitory domain of the calcineurin A alpha protein. A comparison of wild-type and mutant autoinhibitory peptides indicates that this amino acid substitution greatly reduces inhibition of calcineurin phosphatase activity. Additional peptide inhibition studies support a pseudosubstrate model of autoinhibitory function, in which the conserved aspartic acid residue may serve as a molecular mimic of either phosphoserine or phosphothreonine. Expression of the mutant calcineurin appears to affect cellular signal transduction pathways, as EL4 cells can be activated by suboptimal concentrations of calcium ionophore in the presence of phorbol esters. Moreover, this phenotype can be transferred to Jurkat T cells by transfection of the mutated calcineurin gene. These findings implicate a conserved aspartic acid in the mechanism of calcineurin autoinhibition and suggest that mutation of this residue is associated with aberrant calcium-dependent signaling in vivo.

Inhibition of calcineurin phosphatase activity in adult bone marrow transplant patients treated with cyclosporine A

In vitro studies have demonstrated that cyclosporine A (CsA) acts by inhibiting the phosphatase activity of calcineurin, an important mediator of T-cell activation. The relationship of CsA administration in vivo, calcineurin activity, and graft-versus-host disease (GVHD) has yet to be studied. The calcineurin activities of mononuclear cells isolated from 62 bone marrow transplant recipients and 12 normal volunteers were determined and analyzed with respect to administration of CsA, presence or absence of CsA in plasma, and presence or absence of GVHD. Of 62 patients, 33 were taking CsA and 29 were not. Early posttransplant (< 100 days), the calcineurin activity of patients on CsA was significantly lower than that of patients not on CsA (P = .0004) and than that of normal volunteers (P < .0001). Similarly, late posttransplant (> 100 days), the calcineurin activity of patients taking CsA was inhibited compared with normal volunteers (P < .05). The calcineurin activity of patients with acute GVHD who were taking CsA was lower than that of patients on CsA without acute GVHD matched for time posttransplant (P = .02). Calcineurin activity in patients on CsA with chronic GVHD was similar to those without chronic GVHD on drug. In conclusion, calcineurin activity is significantly suppressed by in vivo administration of CsA. The lower calcineurin activity of patients on CsA with acute GVHD suggests that CsA-resistant GVHD is not the result of inadequate suppression of calcineurin activity. These data suggest that if inhibition of calcineurin is the only physiologic target of CsA administration, simply increasing doses of CsA or treatment with other inhibitors of calcineurin, such as FK506, would not be expected to ameliorate GVHD.

Cross-linking CD28 leads to activation of 70-kDa S6 kinase

Proliferation of T lymphocytes in response to antigen/MHC complexes is dependent upon the presence of a co-stimulatory signal; in its absence, T cells are rendered unresponsive to specific antigen CD28 is a T cell surface glycoprotein that acts as a co-stimulatory molecule when combined with signals initiated by the T cell receptor CD3 complex. While the biochemical signaling events following CD28 stimulation are still poorly defined, monoclonal antibodies (mAb) directed against CD28 have been shown to transduce a variety of early signals that are different in the presence of cross-linking antibody or the presence of phorbol 12-myristate 13-acetate (PMA), an activator of protein kinase C (PKC). Stimulation of human T cells with cross-linked anti-CD28 mAb alone resulted in the activation of 70-kDa (p70) S6 kinase, a rapamycin-sensitive serine/threonine kinase that is believed to be important for cell cycle progression. Activation of p70 S6 kinase through CD28 was inhibited by rapamycin. Activation of p70 S6 kinase also increased in response to cross-linked CD3, but followed a more rapid time course than activation via CD2. Cyclosporin A and FK506 had no effect on p70 S6 kinase activity initiated via either pathway. The combination of cross-linked CD28 and cross-linked CD3 had no more than an additive effect on the induction of p70 S6 kinase activity. Thus, recruitment of p70 S6 kinase activity appears to represent a common signal transduction event shared by both the CD28 and CD3 pathways of T cell activation.