A LysM and SH3-domain containing region of the Listeria monocytogenes p60 protein stimulates accessory cells to promote activation of host NK cells.

Listeria monocytogenes (Lm) infection induces rapid and robust activation of host natural killer (NK) cells. Here we define a region of the abundantly secreted Lm endopeptidase, p60, that potently but indirectly stimulates NK cell activation in vitro and in vivo. Lm expression of p60 resulted in increased IFNγ production by naïve NK cells co-cultured with treated dendritic cells (DCs). Moreover, recombinant p60 protein stimulated activation of naive NK cells when co-cultured with TLR or cytokine primed DCs in the absence of Lm. Intact p60 protein weakly digested bacterial peptidoglycan (PGN), but neither muropeptide recognition by RIP2 nor the catalytic activity of p60 was required for NK cell activation. Rather, the immune stimulating activity mapped to an N-terminal region of p60, termed L1S. Treatment of DCs with a recombinant L1S polypeptide stimulated them to activate naïve NK cells in a cell culture model. Further, L1S treatment activated NK cells in vivo and increased host resistance to infection with Francisella tularensis live vaccine strain (LVS). These studies demonstrate an immune stimulating function for a bacterial LysM domain-containing polypeptide and suggest that recombinant versions of L1S or other p60 derivatives can be used to promote NK cell activation in therapeutic contexts.

A new function for LAT and CD8 during CD8-mediated apoptosis that is independent of TCR signal transduction.

The majority (>95%) of thymocytes undergo apoptosis during selection in the thymus. Several mechanisms have been proposed to explain how apoptosis of thymocytes that are not positively selected occurs; however, it is unknown whether thymocytes die purely by "neglect" or whether signaling through a cell-surface receptor initiates an apoptotic pathway. We have previously demonstrated that on double positive thymocytes the ligation of CD8 in the absence of TCR engagement results in apoptosis and have postulated this is a mechanism to remove thymocytes that have failed positive selection. On mature single positive T cells CD8 acts as a co-receptor to augment signaling through the TCR that is dependent on the phosphorylation of the adaptor protein, linker for activation of T cells (LAT). Here, we show that during CD8-mediated apoptosis of double positive thymocytes there is an increase in the association of CD8 with LAT and an increase in LAT tyrosine phosphorylation. Decreasing LAT expression and mutation of tyrosine residues of LAT reduced apoptosis upon crosslinking of CD8. Our results identify novel functions for both CD8 and LAT that are independent of TCR signal transduction and suggest a mechanism for signal transduction leading to apoptosis upon CD8 crosslinking.

Mouse models of non-Hodgkin lymphoma reveal Syk as an important therapeutic target.

We have generated mouse models of non-Hodgkin lymphoma (NHL) that rely on the cooperation between MYC overexpression and B-cell antigen receptor (BCR) signaling for the initiation and maintenance of B-cell lymphomas. Using these mouse models of NHL, we have focused on the identification of BCR-derived signal effectors that are important for the maintenance of NHL tumors. In the present study, we concentrate on Spleen tyrosine kinase (Syk), a nonreceptor tyrosine kinase required to transduce BCR-dependent signals. Using a genetic approach, we showed that Syk expression is required for the survival of murine NHL-like tumors in vitro and that tumor cells deficient in Syk fail to expand in vivo. In addition, a pharmacologic inhibitor of Syk was able to induce apoptosis of transformed B cells in vitro and led to tumor regression in vivo. Finally, we show that genetic or pharmacologic inhibition of Syk activity in human NHL cell lines are generally consistent with results found in the mouse models, suggesting that targeting Syk may be a viable therapeutic strategy.

Evidence that CD8+ dendritic cells enable the development of gammadelta T cells that modulate airway hyperresponsiveness.

Airway hyperresponsiveness (AHR), a hallmark of asthma and several other diseases, can be modulated by gammadelta T cells. In mice sensitized and challenged with OVA, AHR depends on allergen-specific alphabeta T cells; but Vgamma1+ gammadelta T cells spontaneously enhance AHR, whereas Vgamma4+ gammadelta T cells, after being induced by airway challenge, suppress AHR. The activity of these gammadelta T cell modulators is allergen nonspecific, and how they develop is unclear. We now show that CD8 is essential for the development of both the AHR suppressor and enhancer gammadelta T cells, although neither type needs to express CD8 itself. Both cell types encounter CD8-expressing non-T cells in the spleen, and their functional development in an otherwise CD8-negative environment can be restored with transferred spleen cell preparations containing CD8+ dendritic cells (DCs), but not CD8+ T cells or CD8- DCs. Our findings suggest that CD8+ DCs in the lymphoid tissues enable an early step in the development of gammadelta T cells through direct cell contact. DC-expressed CD8 might take part in this interaction.

Ligation of CD8 leads to apoptosis of thymocytes that have not undergone positive selection.

Thymocytes that are not positively selected are said to undergo "death by neglect." We have found that ligation of CD8, either by antibodies or MHC class I molecules, induces apoptosis of CD4(+)CD8+ double-positive (DP) thymocytes. The susceptibility of thymocytes to CD8-mediated apoptosis is developmentally regulated and confined to a subpopulation of DP thymocytes. Stimulation through CD3 protects thymocytes from CD8-mediated apoptosis. We suggest that during thymocyte development, binding of CD8 to MHC class I molecules without T cell receptor engagement induces apoptosis in immature DP thymocytes. Our data are consistent with a model in which thymocytes that do not survive positive selection undergo "death by instruction" instead of death by neglect.

Enumeration, phenotyping, and identification of activation events in conjugates between T cells and antigen-presenting cells by flow cytometry.

Microscopic analysis of T cell-antigen-presenting cell (T cell:APC) interactions at the single cell level has been a powerful, but tedious and subjective, technique. In this paper, we describe a rapid and quantitative method to identify T cell:APC conjugates using succinimidyl ester dyes, which irreversibly label free amine groups on the cell surface. The labeled cell conjugates and subsequent activation events are detected by flow cytometry.