Evaluation of naturally acquired IgG antibodies to a chimeric and non-chimeric recombinant species of Plasmodium vivax reticulocyte binding protein-1: lack of association with HLA-DRB1*/DQB1* in malaria exposed individuals from the Brazilian Amazon.

The development of modular constructs that include antigenic regions targeted by protective immune responses is an attractive approach for subunit vaccine development. However, a main concern of using these vaccine platforms is how to preserve the antigenic identity of conformational B cell epitopes. In the present study we evaluated naturally acquired antibody responses to a chimeric protein engineered to contain a previously defined immunodominant domain of the Plasmodium vivax reticulocyte binding protein-1 located between amino acid positions K435-I777. The construct also includes three regions of the cognate protein (F571-D587, I1745-S1786 and L2235-E2263) predicted to contain MHC class II promiscuous T cell epitopes. Plasma samples from 253 naturally exposed individuals were tested against this chimeric protein named PvRMC-RBP1 and a control protein that includes the native sequence PvRBP123-751 in comparative experiments to study the frequency of total IgG and IgG subclass reactivity. HLA-DRB1 and HLA-DQB1 allelic groups were typed by PCR-SSO to evaluate the association between major HLA class II alleles and antibody responses. We found IgG antibodies that recognized the chimeric PvRMC-RBP1 and the PvRBP123-751 in 47.1% and 60% of the studied population, respectively. Moreover, the reactivity index against both proteins were comparable and associated with time of exposure (p<0.0001) and number of previous malaria episodes (p<0.005). IgG subclass profile showed a predominance of cytophilic IgG1 over other subclasses against both proteins tested. Collectively these studies suggest that the chimeric PvRMC-RBP1 protein retained antigenic determinants in the PvRBP1435-777 native sequence. Although 52.9% of the population did not present detectable titers of antibodies to PvRMC-RBP1, genetic restriction to this chimeric protein does not seem to occur, since no association was observed between the HLA-DRB1* or HLA-DQB1* alleles and the antibody responses. This experimental evidence strongly suggests that the identity of the conformational B cell epitopes is preserved in the chimeric protein.

Analysis of AHG-PRA and ELISA-PRA in kidney transplant patients with acute rejection episodes.

Many centers determined a significant correlation between post-transplant anti-HLA antibodies production and clinical outcome. In order to confirm this correlation and ascertain the sequential appearance of anti-HLA antibodies, we compared the ELISA (ELISA-PRA) and the anti-human globulin enhanced complement-dependent lymphocytotoxicity panel-reactive antibodies test (AHG-PRA) with the occurrence of acute rejection episodes. Thirty patients who underwent kidney transplantation between December 1998 and October 1999 were assayed. One pre-transplant and 10 post-transplant serum samples were tested from each recipient except from one of them who lost his graft on the 1 week post-transplant. The diagnosis of acute rejection episode was based on classical criteria (fever, graft swelling and tenderness, oliguria, weight gain) and a rapid rise in serum creatinine levels, confirmed by an allograft biopsy graded by the Banff working classification. The 322 pre- and post-transplant serum specimens were tested by AHG-PRA methodology and 298 of them by the ELISA-PRA. The agreement coefficient (kappa) for both methodologies was 0.63. There were 27 acute rejection episodes in 19 patients. AHG-PRA results were significantly correlated (Hazard ratio=10.06; P=0.006) with this occurrence. These data were not confirmed with the ELISA-PRA procedure. Our results suggest that a routine post-transplant AHG-PRA test offers an early risk assessment of acute rejection episodes and may be a useful method for monitoring the kidney transplant evolution.