The immunology laboratory (Felsburg, Henthorn) has identified and characterized an X-linked severe combined immunodeficiency (XSCID) in dogs which is due to mutations in the common gamma (gc) subunit of the IL-2, IL-4, IL-7, IL-9 and IL-15 receptors. Canine XSCID, unlike genetically engineered gc-deficient mice, has a clinical and immunologic phenotype virtually identical to human XSCID. It appears species-specific differences exist in the role of the gc and its associated cytokines in mice compared to their role in humans and dogs suggesting gc-deficient dogs may be a more relevant model for studying the role of the gc in man.

A central role for the gc, and the cytokines with which it interacts, in T cell development is demonstrated by the profound T cell defect in human and canine XSCID. However, the fact that limited T cell development does occur in human and canine XSCID suggests that aspects of T cell development can occur independently of a functional gc. Much of the proposed functions of the gc, and its downstream signaling molecules, in lymphoid cells are derived from studies using transfected fibroblasts or immortalized lymphoid cell lines. These studies propose that a functional gc is essential for signal transduction in lymphoid cells in response to those cytokines associated with the gc. It remains to be determined in a biologically relevant system whether signaling can occur in lymphoid cells in the absence of a functional gc. It is becoming clear that the cytokines associated with receptors containing the gc are capable of signaling in non-lymphoid cells through pathways different from those attributed to lymphoid cells. We are actively evaluating whether the limited T cell development observed in XSCID may be due to one or more cytokines signaling through nonconventional signaling pathways that are independent of a functional gc. Since gc-dependent cytokines have differing roles in human and canine B cell development than in the mouse, the XSCID dog is also being used to study the role of these cytokines in B cell development and function.

Bone marrow transplantation (BMT) is presently the only treatment available to cure patients with XSCID, however, the major immunologic problem in human XSCID patients following BMT is the engraftment of few, if any, donor B cells with resultant poor reconstitution of humoral immune function unless they receive pre-transplant chemotherapy (cytoablation). Because all of our transplanted XSCID dogs have engrafted donor B cells and reconstituted normal humoral immune function without the need for cytoablative therapy, we are using this unique model to determine which variable(s) contribute to the successful engraftment of donor B cells thereby eliminating the need for the use of potentially harmful cytotoxic drugs. We are also using the XSCID model to develop and evaluate strategies for in utero bone marrow transplantation.

XSCID dogs represent an ideal large animal pre-clinical model for developing and evaluating strategies for human gene therapy. Current studies include the analysis of transduction, engraftment and functional correction of the immune system in XSCID dogs receiving autologous CD34+ marrow cells transduced with a conventional retroviral vector. We will also be using this model to test novel vectors for the ability to give activation-independent expression in resting T cells and to prevent vector silencing --- two problems in current human clinical gene therapy trials.

Various strategies have been used to attempt to study the human immune system in SCID mice; however, it is evident that the mouse does not provide the necessary microenvironment for providing a stable, functional human immune system. Preliminary studies in XSCID dogs suggest that the canine bone marrow and thymic microenvironments, unlike those in the mouse, are capable of supporting the development of mature, circulating human B and T cells when transplanted with human fetal liver hematopoietic stem cells. We are pursuing studies to document the utility of the XSCID dog as a model for studying human hematopoiesis and immune function (XSCID-hu dog). A major advantage of using the XSCID dog is that, due to its size, it represents an experimental small animal model in which repeated blood samples can be collected in order to study the kinetics of lymphopoiesis, response of the immune system to infectious agents, hematopoietic cancers, and to test therapeutic agents in the same animal over time. The XSCID-hu dog would also be a valuable experimental model in which to test strategies for human gene therapy.