2 ANTIGENS AN ZYGOTES AND OOKINETES OF PLASMODIUM-YOELII AND PLASMODIUM-BERGHEI THAT ARE DISTINCT TARGETS OF TRANSMISSION-BLOCKING IMMUNITY

We have developed transmission-blocking monoclonal antibodies (MAbs) a gainst Plasmodium yoelii 21-kDa (Pys21) and 28-kDa (Pys25) ookinete su rface proteins, These MAbs block infectivity of P. yoelii to Anopheles stephensi. One MAb, 14, cross-reacted by Western blotting with a 28-k Da surface protein (Pbs25) of P. berghei ookinetes and blocked oocyst development, as assayed by direct mosquito feeds on passively immunize d P. berghei-infected mice, In total, we have identified two ookinete surface proteins in P. yoelii one of which is also present in P. bergh ei. The transmission-blocking activity of the anti-Pys25 MAb 4 was com plete and more potent than that of the anti-Pys21 MAb 2. Moreover, Fab fragments of MAb 4 had transmission-blocking activity in mice. In com parison, Fab fragments of MAb 2 did not have detectable transmission-b locking effect, although F(ah')(2) did, Furthermore, MAb 2 and MAb 4 a ppeared to block the in vitro formation and development of zygotes as well.