IMMUNOLOGICAL INVESTIGATION IN NIGERIANS INFECTED WITH CHLOROQUINE RESISTANT PLASMODIUM FALCIPARUM

Abstract

In most populations where malaria is endemic, chloroquine has been used as a first line of treatment against P. falciparum. However, with the emergence of chloroquine resistant P. falciparum (CRPF) malaria in many countries including Nigeria, efforts are being intensified on immune intervention in malaria control. Prospective vaccine candidates are being identified, but the changing antigenic components of the parasite, manifested in each emerging strain, is hampering efforts in immune intervention. Host genetic variation in immune response against some important malaria antigenic entities, is also constituting a serious drawback. Therefore there is the need for the evaluation of chloroquine susceptibility of P. falciparum and seroepidemiological surveys, in respect of immune response to prospective malaria vaccine candidates, in populations where P. falciparum infection is endemic. Isolates from 102 Nigerian children with acute infections were used in evaluating chloroquine sensitivity of P. falciparum using schizont inhibition assay (in vitro) technique. Sixty-two of these patients were drawn from Calabar and 40 from Ibadan. The patients were recruited at the respective Teaching Hospitals. Thirty-three of the patients in Calabar were selected (based on standard criteria) for simultaneous in vivo monitoring of susceptibility of the parasite species to standard doses of chloroquine (C25) using the WHO extended 14-day assessment method. Serum samples of all the patients and of 80 age/sex-matched apparently healthy controls were also analysed for some immunological parameters associated with malaria infection. The results indicate a higher prevalence level of CRPF in Calabar (59.2%) when compared to Ibadan (35.3%), by in vitro methods, with a much lower cumulative Ec99 (effective concentration of chloroquine that inhibits 99% of schizont maturation) in the latter than in the former population (2.4 x 10⁻⁶M Vs 4.6 x 10⁻⁶M). A prevalence level of 53.6% (in vivo) in Calabar was recorded giving a high correlation between in vitro (59.2%) and in vivo methods of assessment used in that population. On the overall, children infected with chloroquine sensitive P. falciparum showed a higher geometric mean parasite density on the day of diagnosis (DO), (P< 0.01 in Calabar and P< 0.05 in Ibadan). Eighty percent of patients and 73.3% of the control subjects were seropositive for total blood stage antigen (ELISA). Higher seroreactivity against these antigens was observed in children infected with CRPf than in those infected with the sensitive strains, suggesting that a marker for CRPf may be embodied within the CRPf strain of the parasite. There was no difference in IFA titres between individuals infected with CRPf and those infected with the chloroquine sensitive strain. Forty-four percent of the patients and 29% of the control subjects were seropositive against circumsporozoite (CS) protein. The meanᴼᴰ₄₉₂ of anti-CS antibody reactivity between the two groups of infected patients studied, both in Calabar and in Ibadan, were not significantly different, indicating that all the patients had apparently similar sporozoite inoculation rates. Mean levels of total serum IgM, IgG and its 4 subclasses, and that of the alternative complement pathway factor B (Bf) were higher in malaria patients as compared to the controls. Differences in each of these parameters were also observed between subjects infected with CRPf and those infected with the sensitive strain, however, these differences were significant only in respect of IgG3 subclass where the mean level was higher in subjects infected with CRPf. Mean serum C3 level was lower but not significantly, but C4 level and CH5O activity were significantly lower in patients than in the controls. Generally, it was observed that serum levels of some of the investigated humoral immune parameters varied slightly from previously reported levels within the same population, thus reflecting possible changing antigenic stimuli with time. The immunological relevance of the varying CRPf prevalence levels in the two geographical populations studied and of the differences in immunological profiles in the two groups of infected patients investigated are evaluated. The findings have potential implications in the prospective malaria immune intervention in Nigeria.