PKCtheta is required for alloreactivity and GVHD but not for immune responses toward leukemia and infection in mice.

When used as therapy for hematopoietic malignancies, allogeneic BM transplantation (BMT) relies on the graft-versus-leukemia (GVL) effect to eradicate residual tumor cells through immunologic mechanisms. However, graft-versus-host disease (GVHD), which is initiated by alloreactive donor T cells that recognize mismatched major and/or minor histocompatibility antigens and cause severe damage to hematopoietic and epithelial tissues, is a potentially lethal complication of allogeneic BMT. To enhance the therapeutic potential of BMT, we sought to find therapeutic targets that could inhibit GVHD while preserving GVL and immune responses to infectious agents. We show here that T cell responses triggered in mice by either Listeria monocytogenes or administration of antigen and adjuvant were relatively well preserved in the absence of PKC isoform theta (PKCtheta), a key regulator of TCR signaling. In contrast, PKCtheta was required for alloreactivity and GVHD induction. Furthermore, absence of PKCtheta raised the threshold for T cell activation, which selectively affected alloresponses. Most importantly, PKCtheta-deficient T cells retained the ability to respond to virus infection and to induce GVL effect after BMT. These findings suggest PKCtheta is a potentially unique therapeutic target required for GVHD induction but not for GVL or protective responses to infectious agents.

Type I IFNs provide a third signal to CD8 T cells to stimulate clonal expansion and differentiation.

In this study, we show that IFN-alpha beta can have a direct role in linking innate and adaptive responses by providing the "third signal" needed by naive CD8 T cells responding to Ag and costimulatory ligands. Stimulation of CD8 T cells in the absence of a third signal leads to proliferation, but clonal expansion is limited by poor survival and effector functions do not develop. We show that IFN-alpha beta can provide the third signal directly to CD8 T cells via a STAT4-dependent pathway to stimulate survival, development of cytolytic function, and production of IFN-gamma. Provision of the third signal by either IFN-alpha beta or IL-12 results in regulation of the expression of a number of genes, including several that encode proteins critical for effector function.

Cutting edge: Bcl-3 up-regulation by signal 3 cytokine (IL-12) prolongs survival of antigen-activated CD8 T cells.

Clonal expansion of T cells requires cell division and survival during the proliferative phase of the response. Naive murine CD8 T cells responding to Ag and costimulation undergo an abortive response characterized by impaired clonal expansion, failure to develop effector functions, and long-term tolerance. A third signal provided by IL-12 is required for full expansion, activation, and establishment of memory. The enhanced survival, and thus clonal expansion, supported by IL-12 is not due to increased Bcl-2 or Bcl-x(L) expression; both are maximally activated by signals 1 and 2. In contrast, Bcl-3, recently shown to enhance survival when ectopically expressed in T cells, is increased only when IL-12 is present. Furthermore, examination of Bcl-3-deficient CD8 T cells demonstrates that the increased survival caused by IL-12 depends upon Bcl-3. The time courses of expression suggest that Bcl-2 and Bcl-x(L) promote survival early in the response, whereas Bcl-3 acts later in the response.