TCR-independent T cell development mediated by gain-of-oncogene function or loss-of-tumor-suppressor gene function

B-cell receptors and heavy chain diseases: guilty by association?

Heavy chain diseases (HCDs) are B-cell proliferative disorders characterized by the production of monoclonal, incomplete, immunoglobulin (Ig) heavy chains (HCs) without associated light chains (LCs). These abnormal HCs are produced as a consequence of HC gene alterations in the neoplastic B cells. HC gene alterations will also impact on surface HC, which is part of the B-cell receptor (BCR), a crucial player in lymphocyte activation by antigen. The selective advantage conferred to mutant cells by abnormal BCR without an antigen-binding domain may be explained by activation of ligand-independent signaling, in analogy to what has been shown for mutated oncogenic growth factor receptors. Here we review data obtained from mouse models showing abnormal, constitutive activity of HCD-BCR, and we discuss the possible mechanism involved, namely, aberrant spontaneous self-aggregation. This self-aggregation might occur as a consequence of escape from the chaperone immunoglobulin binding protein (BiP) and from the anti-aggregation effect of LC association. The concept of misfolding-induced signaling elaborated here may extend to other pathologies termed conformational diseases.

An anti-PR1/HLA-A2 T-cell receptor-like antibody mediates complement-dependent cytotoxicity against acute myeloid leukemia progenitor cells

PR1 (VLQELNVTV) is a human leukocyte antigen-A2 (HLA-A2)-restricted leukemia-associated peptide from proteinase 3 (P3) and neutrophil elastase (NE) that is recognized by PR1-specific cytotoxic T lymphocytes that contribute to cytogenetic remission of acute myeloid leukemia (AML). We report a novel T-cell receptor (TCR)-like immunoglobulin G2a (IgG2a) antibody (8F4) with high specific binding affinity (dissociation constant [K(D)] = 9.9nM) for a combined epitope of the PR1/HLA-A2 complex. Flow cytometry and confocal microscopy of 8F4-labeled cells showed significantly higher PR1/HLA-A2 expression on AML blasts compared with normal leukocytes (P = .046). 8F4 mediated complement-dependent cytolysis of AML blasts and Lin(-)CD34(+)CD38(-) leukemia stem cells (LSCs) but not normal leukocytes (P < .005). Although PR1 expression was similar on LSCs and hematopoietic stem cells, 8F4 inhibited AML progenitor cell growth, but not normal colony-forming units from healthy donors (P < .05). This study shows that 8F4, a novel TCR-like antibody, binds to a conformational epitope of the PR1/HLA-A2 complex on the cell surface and mediates specific lysis of AML, including LSCs. Therefore, this antibody warrants further study as a novel approach to targeting leukemia-initiating cells in patients with AML.

Helix-loop-helix proteins regulate pre-TCR and TCR signaling through modulation of Rel/NF-kappaB activities

E2A and HEB are basic helix-loop-helix transcription factors essential for T cell development. Complete inhibition of their activities through transgenic overexpression of their inhibitors Id1 and Tal1 leads to a dramatic loss of thymocytes. Here, we suggest that bHLH proteins play important roles in establishing thresholds for pre-TCR and TCR signaling. Inhibition of their function allows double-negative cells to differentiate without a functional pre-TCR, while anti-CD3 stimulation downregulates bHLH activities. We also find that the transcription factor NF-kappaB becomes activated in transgenic thymocytes. Further activation of NF-kappaB exacerbates the loss of thymocytes, whereas inhibition of NF-kappaB leads to the rescue of double-positive thymocytes. Therefore, we propose that E2A and HEB negatively regulate pre-TCR and TCR signaling and their removal causes hyperactivation and apoptosis of thymocytes.

Alterations induced by chronic stress in lymphocyte subsets of blood and primary and secondary immune organs of mice

BACKGROUND

The immune system is particularly sensitive to stress. Although acute stress generally has positive effects, chronic stress typically provokes immunosuppression. The elucidation of the mechanisms involved in immunosuppression are of interest for the design of therapeutic approaches to avoid the appearance of stress disorders. This study aimed to investigate chronic stress-induced alterations on lymphocyte subset distribution and percentages. The experiments were performed with C57BL/6 mice subjected to chronic immobilization stress.

RESULTS

Stress caused a marked increase in apoptosis inside the thymus, and a reduction in the total number of thymocytes. Furthermore, the proportion of immature thymocytes declined significantly suggesting that the increased apoptosis mainly affected cells of immature phenotype. In blood, the total number of lymphocytes diminished but not all lymphocyte populations showed the same tendency: while the relative proportion of B cells declined slightly, the relative proportion of circulating CD3+ cells, and particularly some T cell subsets showing an immature phenotype (CD3+PNA+), increased under stress. The spleen and lymph nodes show a marked reduction in cellularity, but the relative proportion of T cells increased, while no change or only a slight reduction was observed in the relative proportion of B cells. Similarly, the relative proportion of T cells increased in bone marrow.

CONCLUSIONS

Detailed data on the alterations of lymphoid cell subsets occurring under immobilization stress, both in the bloodstream and in different lymphoid tissues, are obtained. In general, T cells are more affected than B cells and, in particular, a marked increase in the percentage of a subset of circulating PNA+CD3+ T cells is observed.