Vaccinology, immunology, and comparative pathogenesis of measles in the quest for a preventative against AIDS

Current progress in the development of a prophylactic vaccine for HIV-1

Since its discovery and characterization in the early 1980s as a virus that attacks the immune system, there has been some success for the treatment of human immunodeficiency virus-1 (HIV-1) infection. However, due to the overwhelming public health impact of this virus, a vaccine is needed urgently. Despite the tireless efforts of scientist and clinicians, there is still no safe and effective vaccine that provides sterilizing immunity. A vaccine that provides sterilizing immunity against HIV infection remains elusive in part due to the following reasons: 1) degree of diversity of the virus, 2) ability of the virus to evade the hosts' immunity, and 3) lack of appropriate animal models in which to test vaccine candidates. There have been several attempts to stimulate the immune system to provide protection against HIV-infection. Here, we will discuss attempts that have been made to induce sterilizing immunity, including traditional vaccination attempts, induction of broadly neutralizing antibody production, DNA vaccines, and use of viral vectors. Some of these attempts show promise pending continued research efforts.

Recombinant vector derived from live attenuated measles virus: Potential for flavivirus vaccines

The measles vaccine is one of the best vaccines currently available. Over the last 30 years, it has been administered to hundreds of millions of children and has proved to be both effective and safe. This attenuated live virus induces life-long immunity after only one or two injections. It is produced on a large scale, with ease, in many countries and is distributed at low cost. These excellent characteristics led us to consider its use as a pediatric live vector, to simultaneously immunize children or adolescents against measles and other viral infections, such as human immunodeficiency virus (HIV) or flavivirus infections. For this purpose, we have developed a vector derived from the live attenuated Schwarz strain of the measles virus (MV). We have demonstrated the capacity of this vector to strongly and stably express genes encoding proteins from HIV and to induce specific humoral and cellular immune responses in vivo. Importantly, we observed that, at least in animal models, the vector can bypass measles vaccine pre-existing immunity when two doses of recombinant vaccine are administered. Clinical trials are in progress to confirm this observation in immunized adults. We also produced MV vectors expressing proteins from West Nile virus and other flaviviruses, which in the case of West Nile virus, protected against experimental challenge. Recombinant live attenuated MV might be used as bivalent vaccination vector to mass immunize children and adolescents against both measles and flaviviral diseases such as Dengue or Japanese Encephalitis in the developing world.

Attenuated measles virus as a vaccine vector

Live attenuated measles virus (MV) vaccines have an impressive record of safety, efficacy and ability to induce life-long immunity against measles infection. Using reverse genetics technology, such negative-strand RNA viruses can now be rescued from cloned DNA. This technology allows the insertion of exogenous genes encoding foreign antigens into the MV genome in such a way that they can be expressed by the MV vaccine strain, without affecting virus structure, propagation and cell targeting. Recombinant viruses rescued from cloned cDNA induce immune responses against both measles virus and the cloned antigens. The tolerability of MV to gene(s) insertion makes it an attractive flexible vector system, especially if broad immune responses are required. The fact that measles replication strictly occurs in the cytoplasm of infected cells without DNA intermediate has important biosafety implications and adds to the attractiveness of MV as a vector. In this article we report the characteristics of reporter gene expression (GFP, LacZ and CAT) and the biochemical, biophysical and immunological properties of recombinant MV expressing heterologous antigens of simian immunogeficiency virus (SIV).

Live attenuated measles vaccine as a potential multivalent pediatric vaccination vector

Live attenuated RNA viruses make highly efficient vaccines. Among them is the live attenuated measles virus (MV) vaccine that has been given to a very large number of children and has been shown to be highly efficacious and safe. MV vaccine induces a life-long immunity after a single injection or two low-dose injections. It is easily produced on a large scale in most countries and can be distributed at low cost. Reversion to pathogenicity has never been observed with this vaccine. For all of these characteristics, developing of MV vaccine vector as a multivalent vaccine to immunize children against both measles and other infectious agents such as human immunodeficiency virus (HIV), flaviviruses, or malaria might be very promising for worldwide use. As MV vaccine is inexpensive to produce, the generation of recombinant vaccines may remain affordable and attractive for the developing word. In this article, we describe the development of MV vector and present some recent data showing the capacity of recombinant MV vaccine to express various proteins from HIV and West Nile virus. In addition, the ability of recombinant MV to induce specific immune responses against these different pathogens are presented and discussed.

A paediatric vaccination vector based on live attenuated measles vaccine

Live attenuated RNA viruses make highly efficient vaccines. Among them, measles virus (MV) vaccine has been given to a very large number of children and shown to be highly effective and safe. MV vaccine induces a life-long immunity after a single or two low-dose injections. It is easily produced on a large scale in most countries and can be distributed at low cost. Reversion to pathogenicity has never been observed with this vaccine. Because of all these characteristics, MV vaccine might be a very promising vector to immunise children against both measles and other infectious agents, such as HIV or flaviviruses, in the developing world. In this article, we describe recent data that we obtained showing the capacity of recombinant Schwarz MVs to express proteins from human immunodeficiency or West Nile viruses, and to induce specific immune responses able, in the case of West Nile virus, to protect from an experimental challenge.

Use of community-based participatory research in preparing low income and homeless minority populations for future HIV vaccines

We conducted Community-Based Participatory Research (CBPR), using a qualitative focus group design, to assess factors that might impact participation of high-risk impoverished adults in future HIV Vaccine Trials (HIVVTs). The participants were 40 homeless and low-income adults recruited from subsidized apartments and homeless shelters in Los Angeles. Findings revealed that the participants expressed both concerns and interest in future HIVVTs. Concerns centered on the impact of the vaccine on their physical health, the possibility of seroconverting and its associated stigma. While distrust of the government was pervasive, the participants were interested in receiving more information about the vaccine from the researchers. They also wished to have their voices heard by the researchers early in the design of the vaccines. Motivating factors were also discovered, and included altruism, compensation and access to care. Perception that risk behaviors might increase among some as a result of participation in a future HIVVT was likewise revealed. Implications of the study reveal that while impoverished populations are interested in participating in future HIVVTs, the researchers must address concerns early on. Moreover, the importance of ongoing education and counseling to warn about hazards of engaging in risky behavior while participating in a future HIVVT was critical.

A single injection of recombinant measles virus vaccines expressing human immunodeficiency virus (HIV) type 1 clade B envelope glycoproteins induces neutralizing antibodies and cellular immune responses to HIV

The anchored and secreted forms of the human immunodeficiency virus type 1 (HIV-1) 89.6 envelope glycoprotein, either complete or after deletion of the V3 loop, were expressed in a cloned attenuated measles virus (MV) vector. The recombinant viruses grew as efficiently as the parental virus and expressed high levels of the HIV protein. Expression was stable during serial passages. The immunogenicity of these recombinant vectors was tested in mice susceptible to MV and in macaques. High titers of antibodies to both MV and HIV-Env were obtained after a single injection in susceptible mice. These antibodies neutralized homologous SHIV89.6p virus, as well as several heterologous HIV-1 primary isolates. A gp160 mutant in which the V3 loop was deleted induced antibodies that neutralized heterologous viruses more efficiently than antibodies induced by the native envelope protein. A high level of CD8+ and CD4+ cells specific for HIV gp120 was also detected in MV-susceptible mice. Furthermore, recombinant MV was able to raise immune responses against HIV in mice and macaques with a preexisting anti-MV immunity. Therefore, recombinant MV vaccines inducing anti-HIV neutralizing antibodies and specific T lymphocytes responses deserve to be tested as a candidate AIDS vaccine.

Current overview of the pathogenesis and prophylaxis of measles with focus on practical implications

Measles is one of the most important diseases of mankind, which is so highly contagious and evokes such persistent immunity that the virus cannot be sustained in a population of less than about 500,000 persons. The first of the licensed live virus vaccines against measles was developed empirically and was approved in 1963. It provides high level and lasting immunity and is a paradigm for solving major medical problems without really understanding them. In spite of means for control by prophylactic immunization, research on measles infection continues to be part of the effort to understand the pathogenesis of many different viruses, which may have important similarities and differences and provide important insights. Measles, usually, is spontaneously reversible and is a prime model for understanding virus-induced immunodeficiency disease (AIDS) which is rarely reversible. Much has been learned of basic immunology and vaccinology in measles through observation of the inappropriate use of vaccines of appropriate composition, and through inappropriate host response to measles vaccines of inappropriate composition. This review provides a current overview of selected highlights of measles, the virus, its immunopathogenesis, and its control by use of live virus vaccine which may lead to elimination of the disease and eventually to eradication of the virus.

Apoptotic effects in primary human umbilical vein endothelial cell cultures caused by exposure to virion-associated and cell membrane-associated HIV-1 gp120

During the course of HIV-1 infection, free virus, infected cells, and free HIV-1 proteins circulate within the host, exposing the host endothelium to these viral factors. We have previously presented evidence showing that soluble HIV-1 gp120 protein interacts with chemokine receptors on primary human endothelium and (through those interactions) induces apoptosis as well as other intracellular effects. The current study examines the effect of exposure of vascular endothelium to gp120 IIIb expressed on the surface of Jurkat cells and in the context of viral particles. Apoptosis was observed in human umbilical vein endothelial cell (HUVEC) cultures exposed to gp160-transfected Jurkat cells as well as to virion particles with gp120 on their surface. Additional experiments show that this apoptotic effect was caused by gp120 protein acting through chemokine receptors on the HUVEC surface, primarily the CXCR4 receptor. At higher concentrations of gp120, this lymphotrophic variant, which has been shown to interact predominantly with CXCR4, seems to interact with and induce apoptosis through the CCR5 receptor. Finally, this apoptotic effect in HUVEC cultures occurs at low levels of the inducing agent, gp120, on cell membranes or on virion particles. These results demonstrate that HIV-1 gp120 is capable of interacting with and killing vascular endothelial cells in multiple in vivo contexts.

Involvement of protein kinase C in HIV-1 gp120-induced apoptosis in primary endothelium

We previously showed that HIV-1 gp120-induced apoptosis in primary human umbilical vein endothelial cell cultures (HUVEC), through CCR5 and CXCR4. Here, we have found that agonists of protein kinase C (PKC), basic fibroblast growth factor (bFGF), and short exposure to low concentrations of phorbol esters were found to block gp120-induced apoptosis in HUVEC cultures. PKC antagonists, sphingosine, H7, and extended exposure of cultures to high concentrations of phorbol esters were also found to block gp120-induced apoptosis in HUVEC cultures. A significant increase in the total amount of cellular PKC enzymatic activity was observed on exposure of HUVEC to gp120. No increase in total PKC activity was observed on exposure of HUVECs to the natural ligands SDF-1alpha, or regulated-on-activation normal T-expressed and secreted (RANTES) cells, and gp120-induced PKC induction was found to be totally blocked by CXCR4 antibodies and partially blocked by the caspase 3 inhibitor, DEVD-CHO. Alternatively, CXCR4 antibodies and DEVD-CHO totally blocked apoptosis. Finally, gp120-induced effects were found to be insensitive to pertussis toxin. Accumulated evidence suggests PKC involvement at multiple points in the gp120-induced apoptotic pathway; also suggests involvement of the CXCR4 receptor internalization pathway, and potentially suggests different downstream effects of gp120-receptor interactions and natural ligand-receptor interactions.

A simplified vaccinologists' vaccinology and the pursuit of a vaccine against AIDS

Vaccinology is the science and engineering of developing vaccines to prevent infectious diseases. Guidelines come from knowledge of pathogenesis and from successful past vaccines. The vaccine enterprise relies on the evolution of appropriate science and technology. Governmental support and industrial participation are key to successful development of new vaccines. A large challenge for vaccinology is a vaccine which protects against AIDS. Though misguided in its first decade, current vaccine research is directed to use of any and all viral antigens and to elicit both cell-mediated and humoral immune responses that are resident, with memory, at the mucosal sites of viral entry. Recent seminal discoveries guiding the future include selective elicitation of both Type 1 and Type 2 immune responses, and prime-boosting using recombinant viral or DNA vectors and expressed antigens. Success in vaccinology depends on simplification of the complex and on iterative processes in a well-defined pathway. The present and future of vaccinology are discussed in depth.

Effects of subcutaneous interleukin-2 therapy on CD4 subsets and in vitro cytokine production in HIV+ subjects

HIV infection is characterized by the reduction of the CD4+, CD45RA+, CD26+, and CD28+ lymphocyte subsets and of the in vitro production of IL-2, IL-4, and interferon-gamma; on the contrary, chemokine production is usually increased. These abnormalities are only partially restored by antiretroviral chemotherapy. Therapy with interleukin-2 has been proposed to restore the functions of the immune system, but the mechanisms by which IL-2 exerts its activities are unknown. The aim of this study was to define the effects of rIL-2 administration on CD4+, CD45RA+, CD45R0+, and CD26+ lymphocytes and on the in vitro production of IL-2, IL-4, IL-10, IFN-gamma, RANTES, and sCD30 in HIV+ patients. 10 HIV+ patients with CD4 cell counts between 200 and 500 cells/mm3 were treated with six cycles of subcutaneous recombinant IL-2 administration, in combination with zidovudine and didanosine. This therapeutic regimen resulted in a remarkable increase in the number of CD4+ cells and in the prolonged reduction of the levels of viremia. CD45R01 cells were expanded during the first cycle of therapy, while CD45RA+/CD26+ cells predominated after the third cycle. At this time, the in vitro production of IL-2, IL-4, IFN-gamma, and sCD30 were significantly upregulated. These results demonstrate that rIL-2 in HIV+ patients induces the reconstitution of the CD4/CD45RA lymphocytes subtype. This expanded cell population recovered the ability to produce in vitro IL-2, IL-4, and IFN-gamma. These effects may be beneficial to HIV+ patients by improving their immune response to microorganisms or vaccines.

Interleukin-12 in infectious diseases

Interleukin-12 (IL-12) is a potent immunoregulatory cytokine that is crucially involved in a wide range of infectious diseases. In several experimental models of bacterial, parasitic, viral, and fungal infection, endogenous IL-12 is required for early control of infection and for generation and perhaps maintenance of acquired protective immunity, directed by T helper type 1 (Th1) cells and mediated by phagocytes. Although the relative roles of IL-12 and gamma interferon in Th1-cell priming may be to a significant extent pathogen dependent, common to most infections is that IL-12 regulates the magnitude of the gamma interferon response at the initiation of infection, thus potentiating natural resistance, favoring Th1-cell development; and inhibiting Th2 responses. Treatment of animals with IL-12, either alone or as a vaccine adjuvant, has been shown to prevent disease by many of the same infectious agents, by stimulating innate resistance or promoting specific reactivity. Although IL-12 may enhance protective memory responses in vaccination or in combination with antimicrobial chemotherapy, it is yet unclear whether exogenous IL-12 can alter established responses in humans. Continued investigation into the possible application of IL-12 therapy to human infections is warranted by the role of the cytokine in inflammation, immunopathology, and autoimmunity.

Experimental measles. II. Infection and immunity in the rhesus macaque

Measles infection and the host immune response to measles virus was compared using naive and immunized rhesus monkeys. The monkeys were experimentally challenged with a wild-type strain of measles virus inoculated intranasally. After pathogenic virus challenge, measles virus was detected in mononuclear cells of peripheral blood, lymph node, and spleen in naive monkeys and viremia peaked on Day 7. However, only one of five vaccinated monkeys had a lower virus titer in peripheral blood mononuclear cells at one time point after challenge. No virus was detected in the lymphoid tissues from an immunized monkeys that was euthanized on Day 7 of infection. Measles-specific IgM, IgG, neutralizing antibody, and cytotoxic T lymphocytes were detected in vaccinated monkeys before challenge, but antibody titers were significantly lower in immunized monkeys than in naive monkey after challenge. Measles-specific IgG antibody and cytotoxic T cell responses were still detected more than 1 year after vaccination or infection. This animal model is useful for the further study of measles pathogenesis, immunosuppression, and immunologic memories.

Type 1 and type 2 cytokine dysregulation in human infectious, neoplastic, and inflammatory diseases

In the mid-1980s, Mosmann, Coffman, and their colleagues discovered that murine CD4+ helper T-cell clones could be distinguished by the cytokines they synthesized. The isolation of human Th1 and Th2 clones by Romagnani and coworkers in the early 1990s has led to a large number of reports on the effects of Th1 and Th2 on the human immune system. More recently, cells other than CD4+ T cells, including CD8+ T cells, monocytes, NK cells, B cells, eosinophils, mast cells, basophils, and other cells, have been shown to be capable of producing "Th1" and "Th2" cytokines. In this review, we examine the literature on human diseases, using the nomenclature of type 1 (Th1-like) and type 2 (Th2-like) cytokines, which includes all cell types producing these cytokines rather than only CD4+ T cells. Type 1 cytokines include interleukin-2 (IL-2), gamma interferon, IL-12 and tumor necrosis factor beta, while type 2 cytokines include IL-4, IL-5, IL-6, IL-10, and IL-13. In general, type 1 cytokines favor the development of a strong cellular immune response whereas type 2 cytokines favor a strong humoral immune response. Some of these type 1 and type 2 cytokines are cross-regulatory. For example, gamma interferon and IL-12 decrease the levels of type 2 cytokines whereas IL-4 and IL-10 decrease the levels of type 1 cytokines. We use this cytokine perspective to examine human diseases including infections due to viruses, bacteria, parasites, and fungi, as well as selected neoplastic, atopic, rheumatologic, autoimmune, and idiopathic-inflammatory conditions. Clinically, type 1 cytokine-predominant responses should be suspected in any delayed-type hypersensitivity-like granulomatous reactions and in infections with intracellular pathogens, whereas conditions involving hypergammaglobulinemia, increased immunoglobulin E levels, and/or eosinophilia are suggestive of type 2 cytokine-predominant conditions. If this immunologic concept is relevant to human diseases, the potential exists for novel cytokine-based therapies and novel cytokine-directed preventive vaccines for such diseases.

Candidate HIV type 1 multideterminant cluster peptide-P18MN vaccine constructs elicit type 1 helper T cells, cytotoxic T cells, and neutralizing antibody, all using the same adjuvant immunization

Cytotoxic T lymphocytes and Th1 cells have been suggested to play a critical role in the control of HIV infection. It is therefore considered that a vaccine that induces a strong Th1 response and CTL response would be more efficacious than one that does not in providing protection against infection and progression toward AIDS. In this study we show that immunization with vaccine constructs consisting of multideterminant cluster peptides containing Th epitopes from the HIV-1IIIB envelope colinearly synthesized to peptide 18MN, is capable of inducing a Th1 response in mice and, dependent on this help, both cytotoxic T cell responses and neutralizing antibody toward the homologous strain of HIV. Moreover, the cytotoxic T cell response elicited by immunization with a mixture of cluster peptide-P18MN vaccine constructs was at least as cross-reactive against known viral variant P18 target sequences as a CTL line produced by immunization with a vaccinia construct expressing recombinant gp160 MN. Four adjuvants were compared to optimize both CTL and antibody responses. A single adjuvant formulation of peptide in ISA 51 could elicit all three: Th1 cells, CTLs, and neutralizing antibody. Thus, immunization directed toward the development of a cytotoxic T cell response does not preclude the development of neutralizing antibody and vice versa, i.e., the responses are not mutually exclusive. The immunization protocol described here should be directly applicable for study in clinical trials aimed at HIV-1 immunotherapy or prophylaxis.

HIV preventive vaccines. Progress to date

The major conceptual problem for HIV vaccine development has been the lack of information on immune responses known to correlate with protection against HIV infection in humans. In this regard, studies on the natural history of HIV infection and AIDS, especially of people with apparent resistance to HIV infection and of patients with HIV infection who have long term survival without disease progression, may provide important information for vaccine development. In addition, a major concern for the development of broadly effective vaccines has been the extensive genetic variability which is characteristic of HIV. In spite of these unknowns, the first generation of HIV candidate vaccines has been developed and evaluated. HIV candidate vaccines based on the subunit recombinant envelope concept (gp120 or gp160) have been shown to protect chimpanzees from HIV infection on challenge, and have now been evaluated in humans in phase I and phase II trials. These products are well tolerated, and capable of inducing neutralising antibodies, but not cytotoxic T lymphocytes. A second vaccine concept, currently in phase I trials, is based on live recombinant vectors, especially using poxvirus vectors followed by boosting with subunit recombinant envelope vaccines. This concept is theoretically very attractive because preliminary data suggest that these vaccines induce both humoral and cell-mediated immunity. However, no published information is available on the ability of live recombinant vector vaccines to protect chimpanzees from HIV infection. The next step in HIV vaccine development is to proceed carefully to expanded phase II and phase III trials to assess the protective efficacy of these candidate vaccines in humans. These trials will be extremely complex from the logistical, scientific and ethical points of view, and will require close collaboration between clinical, basic science and behavioural researchers, national and international organisations, and the pharmaceutical industry.

UV-inactivated measles virus stimulates human and mice naive lymphocytes to proliferate in vitro

Ultra violet (UV) light-inactivated measles virus was used for the lymphocyte proliferation assay, where it caused consistently adult human peripheral blood monolymphocytes (PBML) proliferation which was not dependent on the measles virus complement fixation titre in the donor's sera. When the cord blood lymphocytes from newborn baby and spleen lymphocytes from unprimed Balb/c, C57bl and CBA mice were used in the proliferation assay, all the lymphocytes were stimulated by the UV-inactivated measles virus in a dose-dependent manner. These results suggest that: (1) lymphotropic measles virus can also activate lymphocytes in vitro, and (2) this activation appeared to be neither species- nor major histocompatibility complex (MHC)-restricted. These results may have important implications for measles vaccination and the understanding of the immunopathogenesis of measles virus infection.

The hemagglutinin envelope protein of canine distemper virus (CDV) confers cell tropism as illustrated by CDV and measles virus complementation analysis

Measles virus (MV) and canine distemper virus (CDV) are morbilliviruses that cause acute illnesses and several persistent central nervous system infections in humans and in dogs, respectively. Characteristically, the cytopathic effect of these viruses is the formation of syncytia in permissive cells. In this study, a vaccinia virus expression system was used to express MV and CDV hemagglutinin (HA) and fusion (F) envelope proteins. We found that cotransfecting F and HA genes of MV or F and HA genes of CDV resulted in extensive syncytium formation in permissive cells while transfecting either F or HA alone did not. Similar experiments with heterologous pairs of proteins, CDV-F with MV-HA or MV-F with CDV-HA, caused significant cell fusion in both cases. These results indicate that in this expression system, cell fusion requires both F and HA; however, the functions of these proteins are interchangeable between the two types of morbilliviruses. Human-mouse somatic hybrids were used to determine the human chromosome conferring susceptibility to either MV and CDV. Of the 12 hybrids screened, none were sensitive to MV. Two of the hybrids containing human chromosome 19 formed syncytia following CDV infection. In addition, these two hybrids underwent cell fusion when cotransfected with CDV-F and CDV-HA (but not MV-F and MV-HA) glycoproteins by using the vaccinia virus expression system. To discover the viral component responsible for cell specificity, complementation experiments coexpressing CDV-HA with MV-F or CDV-F with MV-HA in the CDV-sensitive hybrids were performed. We found that syncytia were formed only in the presence of CDV-HA. These results support the idea that the HA protein is responsible for cell tropism. Furthermore, while the F protein is necessary for the fusion process, it is interchangeable with the F protein from other morbilliviruses.

CD30, Th2 cytokines and HIV infection: a complex and fascinating link

CD30 is a member of the tumor necrosis factor (TNF)/nerve growth factor (NGF) receptor superfamily, and was originally described as a marker of Hodgkin's and Reed-Sternberg cells in Hodgkin's lymphoma. CD30 is preferentially expressed on CD4+ and CD8+ T-cell clones that produce T helper 2 (Th2)-type cytokines, and is also released in a soluble form by these cells. Elevated serum levels of soluble (s)CD30 have been found in some conditions in which a pathogenic role for Th2 cells has been suggested, such as atopy, Omenn's syndrome, systemic lupus erythematosus, as well as following infection with measles virus or human immuno-deficiency virus (HIV). Here, Gianfranco Del Prete and colleagues suggest a complex and fascinating link between the expression and release of CD30, and the immunopathogenesis of HIV infection.

Comparative biology and pathogenesis of AIDS and hepatitis B viruses: related but different

AIDS (HIV) and hepatitis B viruses are remarkably similar in their sharing of reverse transcription, in their ancestral origins and common genetic elements, and in their modes of transmission. Both are hypermutable and exist as quasispecies due primarily to errors in reverse transcription, though there is severe restriction in the replicative competence of most hepatitis B mutants. They differ in the lack of an integrase in hepatitis B virus and in their pathogenesis in the infected host. HIV survives mainly by antigenic variability, immune evasion, and impairment of immune function though viral regulatory control elements seek to restrict fatal damage to the host. Hepatitis B virus survives primarily by mutation of e antigen/core genes that directly obviates cytotoxic T cell destruction of infected liver cells, or indirectly limits destruction of infected cells through induction of anergy in the cytotoxic T cell response. Most persons infected with hepatitis B virus recover completely while recovery from HIV infection is rare if ever. Hepatitis B is highly preventable by vaccine while HIV vaccine is still seeking a meaningful immunoprophylactic target. AIDS and hepatitis B represent an extreme example, among the viruses of man, in their close similarities but distinct differences. In depth details and perspectives are presented in this review.