Functional dichotomy in natural killer cell signaling: Vav1-dependent and -independent mechanisms

The product of the protooncogene Vav1 participates in multiple signaling pathways and is a critical regulator of antigen-receptor signaling in B and T lymphocytes, but its role during in vivo natural killer (NK) cell differentiation is not known. Here we have studied NK cell development in Vav1-/- mice and found that, in contrast to T and NK-T cells, the absolute numbers of phenotypically mature NK cells were not reduced. Vav1-/- mice produced normal amounts of interferon (IFN)-gamma in response to Listeria monocytogenes and controlled early infection but showed reduced tumor clearance in vivo. In vitro stimulation of surface receptors in Vav1-/- NK cells resulted in normal IFN-gamma production but reduced tumor cell lysis. Vav1 was found to control activation of extracellular signal-regulated kinases and exocytosis of cytotoxic granules. In contrast, conjugate formation appeared to be only mildly affected, and calcium mobilization was normal in Vav1-/- NK cells. These results highlight fundamental differences between proximal signaling events in T and NK cells and suggest a functional dichotomy for Vav1 in NK cells: a role in cytotoxicity but not for IFN-gamma production.

Dissecting NK cell development using a novel alymphoid mouse model: investigating the role of the c-abl proto-oncogene in murine NK cell differentiation

NK lymphocytes participate in both innate and adaptive immunity by their prompt secretion of cytokines including IFN-gamma, which activates macrophages, and by their ability to lyse virally infected cells and tumor cells without prior sensitization. Although these characteristics of NK cells are well documented, little is known about the genetic program that orchestrates NK development or about the signaling pathways that trigger NK effector functions. By crossing NK-deficient common gamma-chain (gammac) and recombinase activating gene (RAG)-2 mutant mice, we have generated a novel alymphoid (B-, T-, and NK-) mouse strain (RAG2/gammac) suitable for NK complementation in vivo. The role of the c-abl proto-oncogene in murine NK cell differentiation has been addressed in hemopoietic chimeras generated using RAG2/gammac mice reconstituted with c-abl-/- fetal liver cells. The phenotypically mature NK cells that developed in the absence of c-abl were capable of lysing tumor targets, recognizing "missing self," and performing Ab-dependent cellular cytotoxicity. Taken together, these results exclude any essential role for c-abl in murine NK cell differentiation in vivo. The RAG2/gammac model thereby provides a novel approach to establish a genetic map of NK cell development.

The common cytokine receptor gamma chain and the pre-T cell receptor provide independent but critically overlapping signals in early alpha/beta T cell development

Intracellular signals emanating from cytokine and antigen receptors are integrated during the process of intrathymic development. Still, the relative contributions of cytokine receptor signaling to pre-T cell receptor (TCR) and TCR-mediated differentiation remain undefined. Interleukin (IL)-7 interactions with its cognate receptor complex (IL-7Ralpha coupled to the common cytokine receptor gamma chain, gammac) play a dominant role in early thymopoiesis. However, alpha/beta T cell development in IL-7-, IL-7Ralpha-, and gammac-deficient mice is only partially compromised, suggesting that additional pathways can rescue alpha/beta T lineage cells in these mice. We have investigated the potential interdependence of gammac- and pre-TCR-dependent pathways during intrathymic alpha/beta T cell differentiation. We demonstrate that gammac-dependent cytokines do not appear to be required for normal pre-TCR function, and that the rate-limiting step in alpha/beta T cell development in gammac- mice does not involve TCR-beta chain rearrangements, but rather results from poor maintenance of early thymocytes. Moreover, mice double mutant for both gammac and pre-Talpha show vastly reduced thymic cellularity and a complete arrest of thymocyte differentiation at the CD44(+)CD25(+) cell stage. These observations demonstrate that the pre-TCR provides the gammac-independent signal which allows alpha/beta T cell development in gammac- mice. Thus, a series of overlapping signals derived from cytokine and T cell receptors guide the process of alpha/beta thymocyte development.