It is becoming clear that an effective antitumor immunity depends on activation of both arms of the immune system, humoral and cellular. In particular, importance of CD8+ T and CD4+ cells in inhibition of tumor growth and eradication of cancer cells has been often emphasized. Cancer vaccines are aimed at inducing tumor-specific immunity by immunizing patients with tumor cells or their antigenic components, such as tumor-associated antigens (TAA). However, most TAA identified so far are self antigens, thus the ability to generate potent T cell-mediated responses against self-antigens is a prerequisite for the validation of cancer vaccines in humans. We have recently hypothesized that chemokines can be used as carriers to deliver vaccines to various subsets of APCs, including immature DCs, utilizing differentially expressed chemokine receptors. We reported that non-immunogenic self ?tumor antigens or weakly immunogenic foreign antigens rendered immunogenic when immunized as fusions with chemokines. The vaccine formulations did not require use of adjuvants and elicited potent therapeutic and antitumor immunity in several syngeneic murine models of B cell lymphoma. More recently, we have hypothesized that the mechanism by which chemokine fusions elicited responses were in their ability to be efficiently processed through MHC. We have subsequently demonstrated that these fusions can indeed efficiently take up and processed and presented utilizing intracellular trafficking MHC class II pathway. Experiments utilizing inhibitors of intracellular trafficking suggest that chemo-attractant fusion proteins, but not antigen alone, were processed and presented through early-late endosomal and Golgi compartments, and stimulate antigen-specific CD4+ T cells both in vitro and in vivo. Chemokine fusion also facilitated the presentation of antigen by DC to an autologous human tumor-specific CD4+ T cell line. This is an important observation not only on the mechanism of chemokine-vaccine delivery, but also it elucidates for the first time the biology and fate of chemokine receptor trafficking and internalization. At present, we have initiated studies to elucidate possibility that chemokine receptors may also utilize the second pathway, i.e. to be presented via MHC class I pathway. This is indirectly supported by the fact that mice immunized with HIV Env fused with MCP-3 or alpha-defensins elicited efficient systemic and mucosal CD8+ CTL responses. Antitumor therapeutic and protective immunity induced by DNA vaccinations with proinflammatory chemokine fusion antigens was also dependent on effector CD8+ cells. Overall, the use of chemokines as vaccine carriers is an efficient and simple strategy to elicit both humoral and cellular responses. Taking advantage of chemokine redundancy, we have demonstrated that viral chemokine fusions were equally potent in inducing protective immunity in vivo, providing a possible strategy to circumvent hypothetical, vaccine-induced anti-host chemokine autoimmunity (for example, by use of viral chemo-attractants in humans). Although, viral chemokine vMIP2- or MC148-based vaccines elicited protective antitumor immunity against syngeneic B cell lymphoma, not every viral chemokine is a carrier as we could not generate immune responses in mice immunized with constructs expressing HHV6-derived chemokine agonist U83 despite its chemotactic properties for the monocytes. It is not clear whether its ineffectiveness is due to the receptor?s inability to be internalized or the poor stability of the fusion protein. The data have both clinical and basic science applications elucidating biology of chemokine receptor internalization and function. A greater understanding of this process may enable us to harness chemokine receptors for delivery of therapeutic compounds and vaccines. Use of xenogeneic chemokines may also enable the development of safer vaccines for humans. The ideal vaccine carrier should be able to target antigen delivery and possibly recruit antigen-presenting cells (APC), while delivering an activation signal to promote adaptive immune responses. Ligands for chemokine receptors expressed on APC may be attractive candidates since they can both target and attract APC. To investigate the requirement for APC recruitment, we utilized a pair of viral chemokines, agonist vMIP-I and antagonist MC148, which induce and suppress chemotaxis, respectively. Chemokine-antigen fusions efficiently delivered a model non-immunogenic tumor antigen to APC for processing and presentation to antigen-specific T cells in vitro. Physical linkage of chemokine and antigen and specific binding of chemokine receptor by the fusion protein were required. Mice immunized with either vMIP-I or MC148 fusion DNA vaccines elicited protection against tumor challenge. Therefore, antitumor immunity can be elicited in vivo in the absence of vaccine-induced chemotaxis, and vaccine efficacy depends primarily on the ability of the vaccine to target antigen delivery to APC for subsequent processing and presentation. The data have both clinical and basic science applications for the development of simple and effective vaccines for cancer and other clinically relevant diseases. Moreover, these data emphasize the importance and potency of antigen targeting for the induction of immune responses.
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