We previously demonstrated that a specialized subset of immature myeloid cells migrate to lymphoid organs as a result of tumor growth or immune stress, where they suppress B and T cell responses to Ags. Although NO was required for suppression of mitogen activation of T cells by myeloid suppressor cells (MSC), it was not required for suppression of allogenic responses. In this study, we describe a novel mechanism used by MSC to block T cell proliferation and CTL generation in response to alloantigen, which is mediated by the enzyme arginase 1 (Arg1). We show that Arg1 increases superoxide production in myeloid cells through a pathway that likely utilizes the reductase domain of inducible NO synthase (iNOS), and that superoxide is required for Arg1-dependent suppression of T cell function. Arg1 is induced by IL-4 in freshly isolated MSC or cloned MSC lines, and is therefore up-regulated by activated Th2, but not Th1, cells. In contrast, iNOS is induced by IFN-γ and Th1 cells. Because Arg1 and iNOS share l-arginine as a common substrate, our results indicate that l-arginine metabolism in myeloid cells is a potential target for selective intervention in reversing myeloid-induced dysfunction in tumor-bearing hosts.

Despite worldwide eradication of naturally occurring variola virus, smallpox remains a potential threat to both civilian and military populations. New, safe smallpox vaccines are being developed, and there is an urgent need for methods to evaluate vaccine efficacy after immunization. Here we report the identification of an immunodominant HLA-A*0201-restricted epitope that is recognized by cytotoxic CD8 ؉ T cells and conserved among Orthopoxvirus species including variola virus. This finding has permitted analysis and monitoring of epitope-specific T cell responses after immunization and demonstration of the identified T cell specificity in an A*0201-positive human donor. Vaccination of transgenic mice allowed us to compare the immunogenicity of several vaccinia viruses including highly attenuated, replication-deficient modified vaccinia virus Ankara (MVA). MVA vaccines elicited levels of CD8 ؉ T cell responses that were comparable to those induced by the replication-competent vaccinia virus strains. Finally, we demonstrate that MVA vaccination is fully protective against a lethal respiratory challenge with virulent vaccinia virus strain Western Reserve. Our data provide a basis to rationally estimate immunogenicity of safe, second-generation poxvirus vaccines and suggest that MVA may be a suitable candidate.

The fact that you can vaccinate a child at 5 years of age and find lymphoid B cells and antibodies specific for this vaccination 70 years later remains an immunologic enigma. It has never been determined how these long-lived memory B cells are maintained and whether they are protected by storage in a special niche. We report that, whereas blood and spleen compart-

Vaccinia viruses engineered to express foreign genes are powerful vectors for production of recombinant proteins. Originating from highly efficacious vaccines securing world-wide eradication of smallpox, the most appealing use of vaccinia vectors is to serve as vaccine delivery system for heterologous antigens. Concerns about the safety of vaccinia virus have been addressed by the development of vectors based on attenuated viruses. One of them, modified vaccinia virus Ankara (MVA) can be considered as current vaccinia virus strain of choice for clinical investigation. Historical development and use of MVA as vaccine against smallpox allowed to establish an extraordinary safety profile. MVA can be used under conditions of biosafety level 1 because of its avirulence and its deficiency to productively grow in human cells. In recent years significant progress has been made with regard to the development of MVA vector technologies. Compared to replication competent vaccinia viruses, MVA provides similar levels of recombinant gene expression even in nonpermissive cells. In animal models, MVA vaccines have been found immunogenic and protective against various infectious agents including immunodeficiency viruses, influenza, parainfluenza, measles virus, flaviviruses, or plasmodium parasites. By now first data from clinical trials are becoming available. In this article we briefly review history of MVA and state-of-the art technologies with regard to generation of recombinant MVA vaccines, and describe the progress to develop MVA vector vaccines against important infectious diseases.

Safety-tested modified vaccinia virus Ankara (MVA) has been established as a potent vector system for the development of candidate recombinant vaccines. The versatility of the vector system was recently demonstrated by the rapid production of experimental MVA vaccines for immunization against severe acute respiratory syndrome associated coronavirus. Promising results were also obtained in the delivery of Epstein-Barr virus or human cytomegalovirus antigens and from the clinical testing of MVA vectors for vaccination against immunodeficiency virus, papilloma virus, Plasmodium falciparum or melanoma. Moreover, MVA is considered to be a prime candidate vaccine for safer protection against orthopoxvirus infections. Thus, vector development to challenge dilemmas in vaccinology or immunization against poxvirus bio-threat seems possible, yet the right choice should be made for a most beneficial use.