Asymptomatic infection in individuals from the municipality of Barcelos (Brazilian Amazon) is not associated with the anti-Plasmodium falciparum glycosylphosphatidylinositol antibody response

Synthetic Antigens Derived from Plasmodium falciparum Sporozoite, Liver, and Blood Stages: Naturally Acquired Immune Response and Human Leukocyte Antigen Associations in Individuals Living in a Brazilian Endemic Area

Peptide vaccine strategies using Plasmodium-derived antigens have emerged as an attractive approach against malaria. However, relatively few studies have been conducted with malaria-exposed populations from non-African countries. Herein, the seroepidemiological profile against Plasmodium falciparum of naturally exposed individuals from a Brazilian malaria-endemic area against synthetic peptides derived from vaccine candidates circumsporozoite protein (CSP), liver stage antigen-1 (LSA-1), erythrocyte binding antigen-175 (EBA-175), and merozoite surface protein-3 (MSP-3) was investigated. Moreover, human leukocyte antigen (HLA)-DRB1* and HLA-DQB1* were evaluated to characterize genetic modulation of humoral responsiveness to these antigens. The study was performed using blood samples from 187 individuals living in rural malaria-endemic villages situated near Porto Velho, Rondônia State. Specific IgG and IgM antibodies and IgG subclasses were detected by enzyme-linked immunosorbent assay, and HLA-DRB1* and HLA-DQB1* low-resolution typing was performed by PCR-SSP. All four synthetic peptides were broadly recognized by naturally acquired antibodies. Regarding the IgG subclass profile, only CSP induced IgG1 and IgG3 antibodies, which is an important fact given that the acquisition of protective immunity appears to be associated with the cytophilicity of IgG1 and IgG3 antibodies. HLA-DRB1*11 and HLA-DQB1*7 had the lowest odds of responding to EBA-175. Our results showed that CSP, LSA-1, EBA, and MSP-3 are immunogenic in natural conditions of exposure and that anti-EBA antibody responses appear to be modulated by HLA class II antigens.

Parasite Carbohydrate Vaccines

Vaccination is an efficient means of combating infectious disease burden globally. However, routine vaccines for the world's major human parasitic diseases do not yet exist. Vaccines based on carbohydrate antigens are a viable option for parasite vaccine development, given the proven success of carbohydrate vaccines to combat bacterial infections. We will review the key components of carbohydrate vaccines that have remained largely consistent since their inception, and the success of bacterial carbohydrate vaccines. We will then explore the latest developments for both traditional and non-traditional carbohydrate vaccine approaches for three of the world's major protozoan parasitic diseases-malaria, toxoplasmosis, and leishmaniasis. The traditional prophylactic carbohydrate vaccine strategy is being explored for malaria. However, given that parasite disease biology is complex and often arises from host immune responses to parasite antigens, carbohydrate vaccines against deleterious immune responses in host-parasite interactions are also being explored. In particular, the highly abundant glycosylphosphatidylinositol molecules specific for Plasmodium, Toxoplasma, and Leishmania spp. are considered exploitable antigens for this non-traditional vaccine approach. Discussion will revolve around the application of these protozoan carbohydrate antigens for vaccines currently in preclinical development.

Numerical Distributions of Parasite Densities During Asymptomatic Malaria

BACKGROUND

Asymptomatic parasitemia is common even in areas of low seasonal malaria transmission, but the true proportion of the population infected has not been estimated previously because of the limited sensitivity of available detection methods.

METHODS

Cross-sectional malaria surveys were conducted in areas of low seasonal transmission along the border between eastern Myanmar and northwestern Thailand and in western Cambodia. DNA was quantitated by an ultrasensitive polymerase chain reaction (uPCR) assay (limit of accurate detection, 22 parasites/mL) to characterize parasite density distributions for Plasmodium falciparum and Plasmodium vivax, and the proportions of undetected infections were imputed.

RESULTS

The prevalence of asymptomatic malaria as determined by uPCR was 27.5% (1303 of 4740 people tested). Both P. vivax and P. falciparum density distributions were unimodal and log normal, with modal values well within the quantifiable range. The estimated proportions of all parasitemic individuals identified by uPCR were >70% among individuals infected with P. falciparum and >85% among those infected with P. vivax. Overall, 83% of infections were predicted to be P. vivax infections, 13% were predicted to be P. falciparum infections, and 4% were predicted to be mixed infections. Geometric mean parasite densities were similar; 5601 P. vivax parasites/mL and 5158 P. falciparum parasites/mL.

CONCLUSIONS

This uPCR method identified most infected individuals in malaria-endemic areas. Malaria parasitemia persists in humans at levels that optimize the probability of generating transmissible gametocyte densities without causing illness.

Autoimmunity, phospholipid-reacting antibodies and malaria immunity

Several questions regarding the production and functioning of autoantibodies (AAb) during malaria infection remain open. Here we provide an overview of studies conducted in our laboratory that shed some light on the questions of whether antiphospholipid antibodies (aPL) and other AAb associated with autoimmune diseases (AID) can recognize Plasmodia antigens and exert anti-parasite activity; and whether anti-parasite phospholipid antibodies, produced in response to malaria, can inhibit phospholipid-induced inflammatory responses and protect against the pathogenesis of severe malaria. Our work showed that sera from patients with AID containing AAb against dsDNA, ssDNA, nuclear antigens (ANA), actin, cardiolipin (aCL) and erythrocyte membrane antigens recognize plasmodial antigens and can, similarly to monoclonal AAb of several specificities including phospholipid, inhibit the growth of P. falciparum in vitro. However, we did not detect a relationship between the presence of anti-glycosylphosphatidylinositol (GPI) antibodies in the serum and asymptomatic malaria infection, although we did register a relationship between these antibodies and parasitemia levels in infected individuals. Taken together, these results indicate that autoimmune responses mediated by AAb of different specificities, including phospholipid, may have anti-plasmodial activity and protect against malaria, although it is not clear whether anti-parasite phospholipid antibodies can mediate the same effect. The potential effect of anti-parasite phospholipid antibodies in malarious patients that are prone to the development of systemic lupus erythematosus or antiphospholipid syndrome, as well as the (possibly protective?) role of the (pathogenic) aPL on the malaria symptomatology and severity in these individuals, remain open questions.