Negative signal transmission through class II molecules on activated T cells

Atopic asthma: differential activation phenotypes among memory T helper cells

BACKGROUND

T cells have been implicated in the pathogenesis of atopic asthma. We have previously shown that memory T helper cells (CD4+CD45RO+) are preferentially activated relative to naïve T helper cells (CD4+CD45RA+) after bronchial allergen challenge. However, specific T helper subpopulations that are activated in atopy and/or asthma remain undefined.

OBJECTIVE

To determine the T helper subpopulations and activation phenotypes relevant to acute and stable asthma that may be common with or distinct from atopy.

METHODS

Two groups of atopic asthmatics (ten acute and nine stable asthmatics) and two non-asthmatic groups (14 non-asthmatic atopics and eight normal non-atopic controls) were analysed. Ten acute asthmatics were assessed in the emergency room during an acute episode (FEV1 43.6% +/- 18.4). Nine stable asthmatics were assessed during a symptom-free period (FEV1 85% +/- 6). Using multiple colour flow cytometry we analysed T cell subpopulations and the expression of IL-2-receptor (IL-2R) and MHC-class II antigens (MHC II) on naïve and memory T helper cells in the peripheral blood of asthmatic and non-asthmatic groups.

RESULTS

Atopic asthmatics (acute and stable) had an increased percentage of memory T helper cells expressing IL-2R compared with normal non-atopics (mean SD 16.1 +/- 6%, 12.4 +/- 2% and 7.7 +/- 1.8%, P < 0.05) but not compared with non-asthmatic atopics (10 +/- 3.5%). Naïve T helper cells had low expression of IL-2R and MHC II in all four groups. MHC II antigen expression was increased in memory T helper cells of asthmatics (acute and stable) compared with normal non-atopics (13.9 +/- 7.5, 10.6 +/- 5 and 4.9 +/- 2.5, P < 0.05) but not compared with non-asthmatic atopics (7.92 4). A novel finding was that IL-2R and the MHC II molecules were mainly expressed in non-overlapping populations and coexpression was found predominantly on memory T helper cells. Asthmatics (acute and stable) had higher proportion of double positive memory T helper cells (IL-2R+MHC II+) compared with both non-asthmatic groups (P < 0.05).

CONCLUSIONS

We demonstrate a differential expression of IL-2R+ and MCH II+ on CD45RO+ T helper cells that would suggest that there are three subsets of activated memory T helper cells in asthmatics. Two non-overlapping IL-2R+ or MHC II+ CD45RO+ T helper cells and a third subpopulation of activated cells that coexpress IL-2R and MHC II (double positives). This latter subpopulation is significantly higher in asthmatics (acute or stable) compared with both non-asthmatic groups, suggesting a specific T helper activation phenotype distinct to atopic asthmatics as compared with atopic non-asthmatics.

Alloantigen-blocking antibodies in sera from highly sensitized uremic patients: antibody class and relationship with lymphocytotoxins

BACKGROUND

Previously, we identified an antimitogenic IgG antibody separated from sera of patients with known kidney transplant chronic rejection. This antibody inhibits individual patients' own unprimed T helper cell responses to alloantigens as well as a third-party mixed lymphocyte response, but does not inhibit autologous unprimed T helper cell proliferation to adherent anti-CD3 antibody. We suggest that the mechanism of inhibitory action is allogeneic-dependent.

METHODS

We used a series of similar experimental designs to test the presence of this antibody in uremic, sensitized patients and have studied its relationship to sensitization as defined by the presence of lymphocytotoxins in four uremic groups: highly sensitized with or without previous graft loss, moderately sensitized with or without graft loss, nonsensitized without previous graft loss, and nonsensitized with graft loss.

RESULTS

(1) Sensitization is associated with the presence of a potent antibody that blocks primary mixed lymphocyte response. Primed cells are less susceptible to its antimitogenic action. (2) The blocking antibody activity is present only in sensitized patients who have IgG lymphocytotoxic activity against the same HLA class I antigens. (3) The blocking activity is unequal in the following order: IgG 3 > IgG 1 > IgG 2. (4) Although IgG 1 and 2 fractions contain lymphocytotoxic activity against HLA class I antigens, the IgG 3 fraction does not.

CONCLUSIONS

The differential effect of IgG antibodies on naive and memory T cells may explain why humeral responses to alloantigens can be maintained in the presence of blocking antibodies.

T cell major histocompatibility complex class II molecules down-regulate CD4+ T cell clone responses following LAG-3 binding

T cell response to its antigen requires recognition by the T cell receptor together with a co-receptor molecule, either CD4 or CD8. Additional molecules have been identified that are capable of delivering the co-stimulatory signals provided by APC. Following T cell priming, a number of T cell activation antigens are expressed that may play a role in the inactivation phase of the T cell response. The lymphocyte activation gene (LAG)-3 protein and its counter-receptors, the major histocompatibility complex (MHC) class II molecules, are such activation antigens whose interaction may result in the down-regulation of the ongoing immune response. To investigate the role of LAG-3/class II molecule interaction, we produced a soluble form of LAG-3 by fusing the extracellular Ig domains of this membrane protein to the constant region of human IgG1 (LAG-3Ig). Here, we show a direct and specific binding of LAG-3Ig to class II molecules on the cell surface. In addition, we show that LAG-3/class II molecule interaction leads to the down-regulation of CD4+ Ag-specific T cell clone proliferation and cytokine secretion. This inhibitory effect is observed at the level of the effector cells and not the APC and is also found with anti-CD3 mAb, PHA + PMA or low-dose IL-2 driven stimulation in the absence of APC. These functional studies indicate that T cell MHC class II molecules down-regulate T cell proliferation following LAG-3 binding and suggest a role for LAG-3 in the control of the CD4+ T cell response.

Expression of HLA-DR in peripheral nerve of amyotrophic lateral sclerosis

To investigate the possibility of local antigen presentation within the peripheral nerve in amyotrophic lateral sclerosis (ALS), cryostat sections of 83 peripheral nerve biopsies were stained for the demonstration of HLA-DR using a monoclonal antibody. Forty samples showed increased expression of HLA-DR in endoneurium. The phenotypic characteristics of the HLA-DR positive cells are chiefly Schwann cells, using S-100 protein as a marker. We did not detect any co-expression between HLA-DR and NF (axons) and HLA-DR and myelin marker. We also detected co-expression between HLA-DR and NGFr in a majority of HLA-DR positive cells. Inflammatory cells were infrequent, being detected only in 11 cases, predominantly around epineurial blood vessels. Motor and sensory nerve biopsies performed simultaneously showed higher expression of HLA-DR in motor nerves in 2 out of 4 patients. The significance of these findings is not clear. The presence of endoneurial cells expressing HLA-DR suggests that an autoimmune mechanism may be involved in ALS having Schwann as the main target.

Post-transcriptional regulation of MHC class II expression in human T cells

Human T lymphocytes are among those cells which are cell surface class II- in the resting state, but can be induced to express class II following treatment with appropriate stimulators. Although resting T cells do not express detectable surface class II, cell surface class II can be detected on purified T cells as early as 30 min following stimulation with PHA and PMA, well before the initiation of DNA synthesis, and the percentage of positive cells gradually increases with time. One hypothesis explaining this very rapid surface expression of class II is that the genes can be regulated post-transcriptionally in T cells. To test this, we used nuclear run-on assays to measure the transcriptional rate of diverse class II genes in resting and activated T cells. Our results demonstrated that transcripts for DR, DP, and DQ could be detected in cells which were neither dividing nor transcribing mRNA for another marker of T cell activation, the IL-2 gene. Northern blot analysis demonstrated low to moderate steady-state levels of DR beta mRNA in these cells. Moreover, treatment of activated T cells with cycloheximide resulted in superinduction of class II for DR, DQ, and DP. These results suggest that resting T cells can transcribe mRNA for class II genes, but that they do not express the protein product on the cell surface in a detectable way until following activation. In addition, they suggest that there may be a protein factor which negatively influences class II levels in T cells. Thus, the regulation of class II in T cells is complex and involves post-transcriptional regulation, at least in part.