In human patients there is convincing evidence that the immune system plays an important role in the control of malignant disease. Therefore, vaccination strategies to boost anti-tumor immunity and generate immunological memory against multiple tumor targets are being investigated to improve the outcome of human cancer patients. One promising approach has been the use of dendritic cells loaded with tumor antigens to stimulate anti-tumor immunity. Dendritic cells (DCs) are professional antigen presenting cells (APCs) that are responsible for activating immune cells known as T cells in vivo. Preclinical and clinical evidence suggest that ex-vivo pulsing of dendritic cells with tumor lysates and re-injection of “loaded” dendritic cells can trigger polyvalent T cell responses against multiple tumor antigens and can overcome aspects of immune incompetence in cancer patients and result in clinical responses. However, DCs do not proliferate and obtaining large numbers of DC precursors required for a therapeutic vaccine from smaller canine and pediatric human patients is challenging. In collaboration with Dr. Robert Vonderheide at the Abramson Family Cancer Research Institute at the University of Pennsylvania, Drs. Mason and Sorenmo at PennVet are evaluating the use of tumor RNA-transfected CD40-activated autologous B lymphocytes (CD40-B) as an alternative APC vaccine to generate anti-tumor immunity in canine patients with spontaneously occurring lymphoma. Canine lymphoma is a common malignancy affecting the white blood cells known as lymphocytes. The disease has an estimated incidence of 30/100,000 dogs per year. Current therapies to treat canine lymphoma utilize various combinations of chemotherapeutic agents that induce remission in 75-85% of dogs. However, regardless of the chemotherapeutic induction protocol, 85% of these dogs will relapse with clinical signs of systemic disease within 10-12 months of diagnosis. Thus there is a clear need for alternative therapies to maintain remission in canine patients with lymphoma. Basic research performed at Penn has shown that many millions of B cells (known as canine CD40-B cells) can be generated from 4-5mls of peripheral blood taken from canine patients with spontaneously occurring lymphoma. Furthermore, these cells can be loaded with antigen-specific RNA and used to induce antigen-specific T cell responses in vitro. The use of genetic material from the tumor (tumor RNA) as the antigenic payload permits an MHC-independent, multiple-antigen targeting approach particularly important in canine tumors where few tumor-associated antigens have been described. These findings were recently published in the journal Gene Therapy [Mason et al. Gene Therapy. 2008 Mar 13 (Epub ahead of print)] and have been translated into the veterinary clinics where tumor-RNA loaded CD40-B cells are now being evaluated in a phase I clinical trial in canine patients with lymphoma.
Preliminary results from this phase I trial indicate that the use of RNA loaded CD40-B cells is safe and efficacious in prolonging clinical remission times in ~ 40% of patients. Studies are now underway to investigate whether the immunological response induced by CD40-B cells differs between responders and non-responders. Importantly, demonstrating the efficacy of this novel approach to induce therapeutic anti-tumor immunity against lymphoma aims to provide proof of principle that cCD40-B cells can be used as an effective cell based vaccine. This will pave the way to using this technology to treat other types of cancer since RNA can be made from any tumor type and be introduced into cCD40-B cells to stimulate anti-tumor immunity against as yet unidentified tumor-associated antigens. This work has been generously funded by the Alliance for Cancer Gene Therapy, the Barry and Savannah Poodle Memorial Fund and the Portuguese Water Dog Foundation.