A dendritic cell perspective on vaccines: The overarching theme of the laboratory is dendritic cell biology, particularly as it pertains to vaccine science. Vaccines are a medical success story. For more than a century, vaccination has successfully prevented many infectious diseases. They do this by inducing immunity or resistance directed against components of microbes termed antigens. Nevertheless, while new kinds of vaccines are required to prevent many infections, most notably AIDS, other vaccines have the potential to prevent and treat other diseases, e.g., to increase resistance to cancer, and to silence unwanted immune responses, as in autoimmunity, allergy and transplantation. Dendritic cells initiate and control the immune system, making them a logical target to identify new principles and practices that will lead to novel vaccines.
Vaccines are comprised of antigens and adjuvants. The antigens are disease-relevant components, such as parts of a microbe or tumor, which are recognized by the T and B lymphocytes of the immune system. The recognition is highly specific for the antigen and leads to the development of protective functions as well as long term memory. Some protective T cells can eliminate infected cells and tumor cells, while B cells make antibodies that can block infection and lead to target cell killing. Memory T and B cells, once they have been induced by a vaccine, allow the patient to resist infection and cancer in a quicker and more effective way, even long after the vaccine has been given. Therefore vaccine antigens can be recognized by the immune system so that it protects against disease in a specific, nontoxic and durable way.
The adjuvants in a vaccine are the substances that work together with antigens to enhance or reduce immunity. For resistance to infection and cancer, adjuvants should enhance immunity; for allergy, autoimmunity and transplantation, adjuvants should silence undesirable immune reactions. Currently, there is only one adjuvant available to enhance immunity in humans, called alum, but the particular kinds of immunity that develop using the alum adjuvant are inappropriate for resistance to many infections and cancer.
There are many different types of immune reactions that T and B lymphocytes can generate. Accordingly, to develop vaccines, the combination of antigens and adjuvants must elicit a particular and appropriate type of immune response to prevent or treat a given medical condition. Yet, often the underlying science has not been figured out, e.g., scientists have not yet solved which kinds of T cells can resist the AIDS virus. Nevertheless it is clear that the immune system makes qualitatively distinct responses against different viruses, bacteria and parasites, and during cancer, allergy, and inflammatory disease.
An understanding of the biology of dendritic cells is vital to creating a vaccine in terms of its antigens and adjuvants in order to elicit the intended immune responses. First, these cells are specialized in capturing antigens so they can be presented to different types of lymphocytes. This usually means the antigens are metabolized or processed by the dendritic cells into fragments, which then attach to molecules encoded by the major histocompatibility complex (MHC). Individual specific clones of T cells then recognize the "presented antigen" (peptide fragments bound to MHC), and begin to multiply; the T cells either develop helper and killer functions to fight infection and tumors or they bring about allergy, inflammation, and transplant rejection. Second, dendritic cells respond or mature upon encountering adjuvants, and this enables them to induce the immune response. By handling antigens and responding to adjuvants, dendritic cells serve as critical intermediaries to make the vaccine "immunogenic" (capable of enhancing resistance) or "tolerogenic" (capable of silencing immunity).
Dendritic cells as vaccine targets: All cells contain an elaborate system of vesicles containing proteins and particles ingested from the environment, including vaccines and infections. This "endocytic system" was elegantly characterized by Zanvil Cohn and James Hirsch, former heads of this laboratory. They studied professional phagocytic cells, the macrophages and neutrophils or "polys", which scavenge and dismantle antigens and infections with vigor. The endocytic system of dendritic cells is dedicated to the uptake and processing of antigens for recognition by immune lymphocytes. Dendritic cells are professionals at salvaging antigenic fragments as peptide-MHC complexes or as lipid-CD1 complexes that can be recognized by T cells. One particular antigen processing function is called "cross presentation". This means the the vaccine is processed to be presented on MHC class I products for recognition by the killer T cells that resist viruses and many other infections as well as cancer cells. A major goal of our laboratory is to identify principles for the cross presentation of vaccines, so that they can generate strong killer or cytotoxic immunity and memory.
Antigen uptake receptors: Receptors are a fundamental part of the innate immune system since they are the first line of recognition of pathogens. Our major emphasis on antigen uptake receptors expressed by dendritic cells began with the identification of the first one found on dendritic cells in vivo, called DEC-205/CD205, which is studied together with Michel Nussenzweig. Following uptake via DEC-205, antigens are processed onto both MHC class I and II molecules. It turns out that dendritic cells express a plethora of other receptors that take up the antigen and/or respond to the adjuvant, but to study these, one has to identify other monoclonal antibodies specific for the receptor, and then genetically alter mice to study receptor function.
Chae Gyu Park, along with Cheolho Cheong and Jae Hoon Choi, are researching a new family of receptors. These are C-type lectins that bind carbohydrate antigens and are called "SIGNs," which they helped to discover. One family member, DC-SIGN/CD209, is expressed on monocyte-derived dendritic cells. DC-SIGN has an endocytic function, and it allows dendritic cells to bind HIV-1 and several other pathogens. The mouse equivalent of DC-SIGN now needs to be studied to get a better view of the function of this lectin in vivo. Another lectin is Langerin/CD207, which is expressed by all Langerhans cells at body surfaces as well as additional dendritic cells in lymphoid tissues. Still another lectin is SIGN-R1/CD209b, which primarily is expressed by subsets of macrophages and recognizes the capsular polysaccharide of Streptococcus pneumoniae and other pathogens. A final receptor we study, in this case by Hiroaki Hemmi, Koji Suda and Juliana Idoyaga, is called treml4. It is not a lectin, and is expressed on select groups of macrophages and dendritic cells.
Targeting vaccine antigens to dendritic cells: Receptor mediated uptake is being studied in tissue culture and in vivo systems. With the labs of Michel Nussenzweig and Jeffrey Ravetch, we are pursuing a new experimental approach that also could serve as a new approach to vaccination. The approach is to genetically introduce relevant antigens into the heavy chain polypeptide of antibodies that bind to mouse or human/monkey DEC-205 (or antibodies to other dendritic cell receptors). The hybrid antibody then selectively delivers the vaccine antigen to the corresponding receptor on the dendritic cell in vivo, i.e., in mice, monkeys and as we will soon test, people. In mice, the targeting of antigens within antibodies to dendritic cell receptors enhances the efficiency of antigen presentation more than one-hundred fold. Moreover, this approach allows scientists to study receptor and dendritic cell function in the intact animal and patient.
Many of the antigens that we are currently delivering within antibodies to dendritic cells are microbial. The emphasis is on the AIDS virus, especially the HIV gag protein. This research is being carried out by Christine Trumpfheller, Godwin Nchinda, Marina Caskey, Olga Mizenina, Maria Paula Longhi, Scott Barbuto, and Leonia Bozzacco. We are also studying antigens from other infections: proteins from Plasmodia, the protozoa that cause malaria, with Michel Nussenzweig; Yersinia pestis, the bacteria that caused the "black or bubonic plague", with Yoonkyung Do and Chae Gyu Park; the Epstein Barr virus that causes a persistent infection of most humans and in some, lymphomas, with Christian Münz, Cagen Gurer and Till Stowig; and the protozoan parasite, Leishmania major, with Ines Matos.
The immune system very likely needs to generate different types of immune responses to resist each type of microbial infection above. For example, the Leishmania major parasite has served as a valuable model to understand the development of two different kinds of T cell immunity: the Th1 type of CD4+ helper T cell that protects, and the Th2 type that makes individuals more susceptible. When Leishmania major antigen is delivered via the DEC-205 receptor, mice develop particularly strong Th1 type of immunity, and this proceeds via a distinct activation pathway that requires the TNF family member, CD70. Therefore the aim of this research in infectious diseases is to identify principles that enable the immune system to bring about so many different kinds of responses.
A variant of this "antigen targeting" approach is to determine whether other existing types of vaccines can be targeted to dendritic cells, so that vaccine immunity will develop more efficiently in quantitative and qualitative terms. One example are DNA vaccines in which DNA encoding a vaccine antigen is injected into muscle, and then the body develops immunity to the expressed antigen. However DNA vaccines have to date proven to be weak in efficacy. Our research posits that this is because the vaccine antigens are not gaining sufficient access to dendritic cells. Godwin Nchinda is addressing this proposal by encoding dendritic cell targeting sequences into DNA vaccines.
New reseach has been begun by Bei Wang and Jules Cohen to introduce cancer antigens into dendritic cell vaccines to enhance resistance to tumors. Also Sayuri Yamazaki, Niroshana Anandasabapathy and Svetlana Mojsov are targeting self or "autoantigens" to dendritic cells. They seek principle and practices that will allow vaccines to specifically silence or tolerize the immune system, as in juvenile diabetes and multiple sclerosis, where the immune system mistakingly attacks the body.
Vaccine adjuvants. In addition to research on the directed delivery of vaccine antigens to dendritic cells, the laboratory is emphasizing the identification of adjuvants that will induce the dendritic and other cells to generate the appropriate immune response. We are doing this in mice, as well as in monkeys through collaborations with colleagues at the NIH (Robert Seder) and in Germany (Paul Racz, Klara Tenner Racz, Klaus Uberla, Christiane Stahl Hennig, Ralf Ignatius). At the same time, Gaelle Breton and Lillie Cohn are studying cell culture systems to understand adjuvant action on human dendritic cells, while Sarah Schlesinger is leading our program to identify new adjuvants that can be brought into the clinic (see below).
Most adjuvants under current study are either microbial in origin or are designed to mimic microbes. For example, a synthetic form of viral double stranded RNA, termed poly IC, is one of the first adjuvants we are pursuing, trying to understand its distinctive biological properties and its potential for safe use in humans. Dendritic cell maturation, which is coaxed by several known microbial adjuvants, is also mediated by natural body constituents including other lymphocytes like the innate NK and NKT cells. NKT cells recognize glycolipids, including potential nontoxic adjuvants for humans, and these are studied together with Shin-ichiro Fujii and Kanako Shimizu in Japan. The identification of adjuvants, like the identification of relevant antigens, are the lynchpins of a new era of vaccine science, but in both cases, we want to learn how these antigens and adjuvants impinge on the critical intermediaries, dendritic cells.
Dendritic cell subsets. To make DC biology even more intricate, it turns out that there are different types of dendritic cells. Each subset can generate innate and adaptive immune responses, but each can express distinct receptors for antigens and adjuvants. Juliana Idoyaga and Maggi Pack are approaching dendritic cell subsets in mice and in humans, concentrating on lymphoid organs where immune resistance and tolerance are initiated. At the same time, we are characterizing dendritic cells in two vital tissues that have received relatively little attention. Jae Hoon Choi is examining dendritic cells in heart valves and the aorta, while Niroshana Anandasabapathy, Kang Liu and Karen Bulloch are examining the brain. Just as the immune lymphocytes are of many forms, there are also many forms of dendritic cells and many environments in which they operate.
Immune tolerance. While dendritic cells provide important innate and adaptive resistance mechanisms to infections, these cells are now known to present harmless proteins from our own cells ("self") and the environment. In so doing, dendritic cells can actively silence or "tolerize" the immune system. The newly recognized tolerizing roles of dendritic cells, analyzed together with Jeffrey Ravetch and Michel Nussenzweig at Rockefeller, Kayo Inaba in Kyoto, and Mannikam Suthanthiran at Weill Cornell, are regarded as essential counterparts to the better-known functions of dendritic cells in resistance. The concept is that dendritic cells in the steady state, in the absence of acute infection, actively silence the immune system to those self and environmental proteins that they capture. As a result when infection strikes, the dendritic cells have already primed the immune system not to attack harmless targets and they can now focus the immune response to the pathogen.
Dendritic cells prove to be specialized antigen presenting cells for the foxp3+ form of suppressor or regulatory T cell. The latter, better known as "T reg", can suppress virtually all types of immune response. Sayuri Yamazaki and Uri Sela are studying the development and expansion of antigen-specific T reg as a means to control autoimmunity and transplant rejection.
Clinical studies. Diseases in patients continue to inform the spirit and substance of our research. Yet, the causes of human disease can differ considerably from mouse models, and the control of the immune system in humans is largely an unchartered frontier. The methods and concepts from immunology and dendritic cell biology are ready to be extended into the clinic where patients set the standards for the knowledge needed to understand many aspects of diseases and their treatments. To carry out the proof of our concepts, we are using dendritic cells to study and design treatments that can harness the immune system, either to enhance or silence its functions, in an antigen or disease specific manner. Phase I clinical studies of new approaches to vaccination in our hospital are scheduled to begin in 2008-2009 under the direction of Dr. Sarah Schlesinger. We benefit enormously from the support of the Gates Foundation through its Grand Challenges in Global Health program and its sponsorship of Centers for AIDS Vaccine Discovery, ours being led by Dr. David Ho.