African green monkeys (genus Chlorocebus) can be infected with SIVagm, but do not develop AIDS. This natural host of SIV, like sooty mangabeys, maintains high levels of SIV replication but has evolved to avoid immunodeficiency. Elucidating the mechanisms that allow the natural hosts to co-exist with SIV without overt disease may provide crucial information to understand AIDS pathogenesis. Here we show: (1) many CD4+ T cells from African green monkeys down-regulate CD4 in vivo as they enter the memory pool, (2) down regulation of CD4 by memory T cells is independent of SIV infection, (3) the CD4− memory T cells maintain functions which are normally attributed to CD4 T cells including production of IL-2, production of IL-17, expression of FoxP3 and expression of CD40L (4) loss of CD4 expression protects these T cells from infection by SIVagm in vivo, and (5) these CD4− T cells can maintain MHC-II restriction. These data demonstrate that the absence of SIV-induced disease progression in natural hosts species may be partially explained by preservation of a subset of T cells that maintain CD4 T cell function while being resistant to SIV-infection in vivo.

A number of leishmaniasis vaccine candidates are at various stages of pre-clinical and clinical development. Leishmaniasis is a vector-borne neglected tropical disease (NTD) caused by a protozoan parasite of the genus Leishmania and transmitted to humans by the bite of a sand fly. Visceral leishmaniasis (VL, kala-azar) is a high mortality NTD found mostly in South Asia and East Africa, while cutaneous leishmaniasis (CL) is a disfiguring NTD highly endemic in the Middle East, Central Asia, North Africa, and the Americas. Estimates attribute 50,000 annual deaths and 3.3 million disability-adjusted life years to leishmaniasis. There are only a few approved drug treatments, no prophylactic drug and no vaccine. Ideally, an effective vaccine against leishmaniasis will elicit long-lasting immunity and protect broadly against VL and CL. Vaccines such as Leish-F1, F2 and F3, developed at IDRI and designed based on selected Leishmania antigen epitopes, have been in clinical trials. Other groups, including the Sabin Vaccine Institute in collaboration with the National Institutes of Health are investigating recombinant Leishmania antigens in combination with selected sand fly salivary gland antigens in order to augment host immunity. To date, both VL and CL vaccines have been shown to be cost-effective in economic modeling studies.

Highlights► Human hookworm infection is a leading cause of iron deficiency anemia. ► An estimated 700 million people in developing countries are affected. ► The Sabin Vaccine Institute PDP is developing the vaccine in collaboration with FIOCRUZ. ► The vaccine comprises two recombinant protein antigens on alum and a TLR4 agonist. ► The partnership's plan is that the vaccine will be licensed by 2020.

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Several candidate human schistosomiasis vaccines are in different stages of preclinical and clinical development. The major targets are Schistosoma haematobium (urogenitial schistosomiasis) and Schistosoma mansoni (intestinal schistosomiasis) that account for 99% of the world's 252 million cases, with 90% of these cases in Africa. Two recombinant S. mansoni vaccines - Sm-TSP-2 and Sm-14 are in Phase 1 trials, while Smp80 (calpain) is undergoing testing in non-human primates. Sh28GST, also known as Bilhvax is in advanced clinical development for S. haematobium infection. The possibility remains that some of these vaccines may cross-react to target both schistosome species. These vaccines were selected on the basis of their protective immunity in preclinical challenge models, through human immune-epidemiological studies or both. They are being advanced through a combination of academic research institutions, non-profit vaccine product development partnerships, biotechnology companies, and developing country vaccine manufacturers. In addition, new schistosome candidate vaccines are being identified through bioinformatics, OMICs approaches, and moderate throughput screening, although the full potential of reverse vaccinology for schistosomiasis has not yet been realized. The target product profiles of these vaccines vary but many focus on vaccinating children, in some cases following mass treatment with praziquantel, also known as vaccine-linked chemotherapy. Several regulatory pathways have been proposed, some of which rely on World Health Organization prequalification.

Dengue virus is the causative agent of dengue fever and the more-severe dengue hemorrhagic fever (DHF). Human studies suggest that the increased risk of DHF during secondary infection is due to immunopathology partially mediated by cross-reactive memory T cells from the primary infection. To model T cell responses to sequential infections, we immunized mice with different sequences of dengue virus serotypes and measured the frequency of peptide-specific T cells after infection. The acute response after heterologous secondary infections was enhanced compared with the acute or memory response after primary infection. Also, the hierarchy of epitope-specific responses was influenced by the specific sequence of infection. Adoptive-transfer experiments showed that memory T cells responded preferentially to the secondary infection. These findings demonstrate that cross-reactive T cells from a primary infection alter the immune response during a heterologous secondary infection.

Chagas disease, caused by Trypanosoma cruzi, results in an acute febrile illness that progresses to chronic chagasic cardiomyopathy in 30% of patients. Current treatments have significant side effects and poor efficacy during the chronic phase; therefore, there is an urgent need for new treatment modalities. A robust T H 1-mediated immune response correlates with favorable clinical outcomes. A therapeutic vaccine administered to infected individuals could bolster the immune response, thereby slowing or stopping the progression of chagasic cardiomyopathy. Prior work in mice has identified an efficacious T. cruzi DNA vaccine encoding Tc24. To elicit a similar protective cell-mediated immune response to a Tc24 recombinant protein, we utilized a poly(lactic-co-glycolic acid) nanoparticle delivery system in conjunction with CpG motif-containing oligodeoxynucleotides as an immunomodulatory adjuvant. In a BALB/c mouse model, the vaccine produced a T H 1-biased immune response, as demonstrated by a significant increase in antigen-specific IFNg-producing splenocytes, IgG2a titers, and proliferative capacity of CD8 C T cells. When tested for therapeutic efficacy, significantly reduced systemic parasitemia was seen during peak parasitemia. Additionally, there was a significant reduction in cardiac parasite burden and inflammatory cell infiltrate. This is the first study demonstrating immunogenicity and efficacy of a therapeutic Chagas vaccine using a nanoparticle delivery system.

Ascaris lumbricoidesThe Sabin Vaccine Institute Product Development Partnership is developing a Pan-anthelmintic vaccine that simultaneously targets the major soil-transmitted nematode infections, in other words, ascariasis, trichuriasis and hookworm infection. The approach builds off the current bivalent Human Hookworm Vaccine now in clinical development and would ultimately add both a larval Ascaris lumbricoides antigen and an adult-stage Trichuris trichiura antigen from the parasite stichosome. Each selected antigen would partially reproduce the protective immunity afforded by UV-attenuated Ascaris eggs and Trichuris stichosome extracts, respectively. Final antigen selection will apply a ranking system that includes the evaluation of expression yields and solubility, feasibility of process development and the absence of circulating antigen-specific IgE among populations living in helminth-endemic regions. Here we describe a five year roadmap for the antigen discovery, feasibility and antigen selection, which will ultimately lead to the scale-up expression, process development, manufacture, good laboratory practices toxicology and preclinical evaluation, ultimately leading to Phase 1 clinical testing.