Age-dependent association between IgG2 and IgG3 subclasses to Pf332-C231 antigen and protection from malaria, and induction of protective antibodies by sub-patent malaria infections, in Daraweesh.

The certainty of the protective role of acquired immunity in malaria is the major drive for malaria vaccine development. In this study, we measured the levels of total IgG and IgG subclasses to four candidate malaria vaccine antigens; MSP2-3D7, MSP2-FC27, AMA-1 and Pf332-C231, in plasma obtained from a cohort of 136 donors from Daraweesh in Sudan. The cohort was followed for malaria infection for 9 years. After an initial analysis, the immune response to Pf332-C231 antigen was the only one found associated with protection, thus taken for further analysis. The number of previous clinical malaria episodes experienced by the donors was used as an index for relative protection. The number of these episodes was found to be negatively correlated with the levels of pre-existing total IgG, IgG2 and IgG3 to Pf332-C231 (correlation coefficient, CC - 0.215, p=0.012; CC - 0.195, p=0.023 and CC - 0.211, p=0.014, respectively), and also with age (CC - 0.311, p<0.001). Unexpectedly, equal levels of Pf332-C231 antibodies were induced by both patent and sub-patent infections regardless of the number of previous malaria episodes (1-7). Combining the correlation analysis with a multi-linear regression, three variable markers for protection were emerged, two age-dependent, the antibody response to Pf332-C231 and an unidentified marker (likely immune response to other antigens), and the third was an age-independent unidentified marker (possibly gene polymorphisms). In conclusion, this report suggests a protective effect for IgG subclasses to Pf332-C231 antigen against malaria.