Partial T cell activation with an altered superantigenic ligand

Human Flt-3-ligand-mobilized dendritic cells require additional activation to drive effective immune responses

OBJECTIVE

Dendritic cells (DCs) play a pivotal role in the induction of immunity in response to pathogenic challenge or vaccination. As such, the fms-like tyrosine kinase 3-ligand (Flt-3L) has been used to increase DC populations in vivo, with contrasting outcomes, which include an increase in immunity, tolerance induction, or expansion of regulatory cells. This study examines the adjuvant role that human Flt-3L (hFL) administration has in generating immune responses upon immunization with a poorly immunogenic and soluble protein antigen.

MATERIALS AND METHODS

Mice were immunized with the nominal antigen, ovalbumin, alone or with antigen emulsified in complete Freund's adjuvant (CFA), with or without prior hFL-mediated expansion of DC subsets. The maturation of DC subsets and activation status of antigen-specific T cells were analyzed by flow cytometry, with effector function assessed in cytolytic T-lymphocyte assays.

RESULTS

hFL treatment expanded both conventional DC and plasmacytoid DC in vivo, resulting in increased antigen presentation by both direct and cross-presentation pathways. However, it was only in the context of CFA that antigen immunization could mature DCs and subsequently fully activate antigen-specific T cells with enhanced cytolytic activity.

CONCLUSIONS

Our studies reveal that hFL essentially acts as a coadjuvant, as hFL augments the size of an immune response but requires further adjuvant activation to alter the quality of the response.

Anti-immunoglobulin binding and activation of snapper (Pagrus auratus) leucocytes

In order to perform specific immunological assays we have produced and characterised three monoclonal antibodies (MAbs) that bind snapper (Pagrus auratus, Bloch and Schneider) immunoglobulin (Ig). Hybridomas were produced and screened for anti-Ig production using ELISA, Western blot and flow cytometry. All three MAbs (designated 2C5, 4A2 and 1C6) bound specifically to the heavy (H) chain of reduced Ig in Western blot. Furthermore, 1C6 was shown to bind to reduced skin mucus Ig H chain and all three MAbs cross-reacted with the H chain of Atlantic salmon and rainbow trout Ig. In flow cytometric analyses 2C5 and 4A2 bound to B cell populations in the peripheral blood and lymphoid organs. Furthermore, cross-linked 2C5 induced an increase in intracellular protein tyrosine phosphorylation in peripheral blood lymphocytes. Phosphorylated proteins exhibited similar molecular weights to those of mammalian Igalpha and Igbeta and may represent snapper mIg accessory molecule analogues. These data exhibit the potential use of 2C5, 4A2 and 1C6 in both cellular and biochemical analyses of populations of snapper leucocytes.

Structural and functional role of threonine 112 in a superantigen Staphylococcus aureus enterotoxin B

Bacterial superantigens are potent T-cell stimulatory protein molecules produced by Staphylococcus aureus and Streptococcus pyogenes. Their superantigenic activity can be attributed to their ability to cross-link major histocompatibility complex class II molecules with T-cell receptors (TCRs) to form a tri-molecular complex. Each superantigen is known to interact with a specific V(beta) element of TCR. Staphylococcal enterotoxin B (SEB, a superantigen), a primary cause of food poisoning, is also responsible for a significant percentage of non-menstrual associated toxic shock syndrome in patients with a variety of staphylococcal infections. Structural studies have elucidated a binding cavity on the toxin molecule essential for TCR binding. To understand the crucial residues involved in binding, mutagenesis analysis was performed. Our analysis suggest that mutation of a conserved residue Thr(112) to Ser (T112S) in the binding cavity induces a selective reduction in the affinity for binding one TCR V(beta) family and can be attributed to the structural differences in the native and mutant toxins. We present a detailed comparison of the mutant structure determined at 2.0 A with the previously reported native SEB and SEB-TCR V(beta) complex structures.

Altered superantigenic ligands demonstrate the quantitative nature of T-cell activation

In a recent study, a superantigen mutated in the TCR binding site (staphylococcal enterotoxin B (SEB)delta61Y) was described, which behaved as a partial agonist for a Vbeta17-expressing T-cell clone. Evidence is now presented to demonstrate that there is distinct heterogeneity in the response of primary T cells to this protein. Some Vbeta17 T cells responded to SEBdelta61Y by modulating surface receptor expression consistent with activation, and by proliferating. Other Vbeta17 T cells did not proliferate, nor did they display a receptor expression phenotype consistent with activation. However, when repeatedly exposed to the altered superantigen, some of these non-responders entered cell cycle. This pattern of responses was not recapitulated by providing additional costimulation via CD28, although such treatment did induce some of the 'unresponsive' Vbeta17 T cells to upregulate the IL-2 receptor, indicative of partial activation. It was also found that the heterogeneous pattern could be replicated using very low doses of native SEB. The data are discussed in the context of models of T-cell activation in which differences in TCR ligand affinity and dose determine qualitatively different response phenotypes.