Parasitological and immunological effects induced by immunization of Mandrillus sphinx against the human filarial Loa loa using infective stage larvae irradiated at 40 Krad.

Six mandrills were immunized with 150 Loa loa infective stage larvae (L3) irradiated with 40 Krad, and challenged with 100 L3, 60 days after initial vaccination. The parasitological outcome of this immunization was compared to results from six mandrills infected with normal L3. No clear association was seen between vaccination and microfilaremia until day 245 when a significant drop in the level of microfilaria occurred in vaccinated compared to infected animals (5 vs 10 mf/ml; p = 0.012). A one-year follow-up of the humoral immune response showed a strong adult, microfilariae (Mf) and L3 specific IgG response, with distinct profiles for each extract. In immunized animal a significant decrease in antibody level was systematically observed between days 90-145 for the anti-L3 and anti-adult IgG. However, in the same group anti-Mf antibody levels that peaked around 160-175 days post-challenge, were inversely correlated with the decrease in Mf density between day 200 and day 386. These results suggest that immunization with irradiated L3 using these specific conditions may affect the appearance of Mf.

Expression of filarial-specific IgG subclasses under different transmission intensities in a region endemic for loiasis.

Specific IgG subclasses were investigated in two villages (Okoumbi and Ndjokaye) in southeast Gabon with different Loa loa transmission intensities of approximately 9,000 and 1,300 infective larvae (L3) per person per year, respectively. IgG subclasses were measured by an enzyme-linked immunosorbent assay (ELISA) using extracts of L. loa L3, microfilariae (MF), or adult worms. Levels of L3-specific IgG3 were significantly higher in the village with low transmission (Ndjokaye) (P = 0.006). In contrast, MF-specific IgG2 was significantly higher in Okoumbi than in Ndjokaye (P = 0.0009). In the high-transmission village (Okoumbi), levels of both MF- and adult-specific IgG4 were significantly increased in MF carriers compared with amicrofilaremic subjects (P = 0.0015 and P = 0.003, respectively), while levels of L3- and MF-specific IgG1 were significantly higher in amicrofilaremic individuals compared with MF carriers (P = 0.04 and P = 0.03, respectively). Furthermore, among microfilaremic individuals, the level of the specific IgG1 subclass was much lower in Okoumbi than in Ndjokaye (P = 0.036). These results suggest that the expression of antigen-specific IgG3 and IgG2 is more likely to vary with transmission intensity, whereas antigen-specific IgG4 and IgG1 varies with adult worm and MF burden.

The relationship between parasitological status and humoral responses to Loa loa antigens in the Mandrillus sphinx model after immunization with irradiated L3 and infection with normal L3.

In order to identify antigens associated with protection and those associated with active infection, the humoral immune response of 6 Mandrillus sphinx immunized with 150 irradiated L3 and challenged with 100 normal L3 of Loa loa or 6 animals infected with 100 L3 were compared. The plasma of these animals was analysed by Western blot using adult, Mf and L3 antigens. Several antigens with molecular weights varying from 120 kDa to 13 kDa were recognized by the plasma of all animals. It was shown that early recognition of microfilarial antigens with molecular weights of 97, 68, 45 and 33 kDa correlated with the amicrofilaraemic state. A total of 83% of animals with circulating microfilariae had antibodies against the microfilariae 21 kDa antigen. Furthermore, the antibodies against the 21 kDa appeared 1 month before detection of microfilariae in the peripheral blood of 80% of these animals, and declined when animals became amicrofilaraemic. In contrast, when L3 antigen was used, a molecule with a relative molecular weight of 20 kDa was recognized by antibodies of the only animal which remained amicrofilaraemic for 1 year after immunization with irradiated L3. These results suggest that the microfilarial molecule of 21 kDa may be useful as a marker of Loa loa patent infection, whereas the 97, 68, 45 and 33 kDa molecules of microfilariae and the L3 molecule of 20 kDa may be associated with resistance against Loa loa.