To initiate invasion of the mosquito midgut, Plasmodium ookinetes secrete
chitinases that are necessary to cross the
chitin-containing peritrophic matrix en route to invading the epithelial cell surface. To investigate
chitinases as potential immunological targets of blocking
malaria parasite transmission to mosquitoes, a
monoclonal antibody (MAb) was identified that neutralized the enzymatic activity of the sole
chitinase of Plasmodium falciparum,
PfCHT1, identified to date. This MAb, designated 1C3, previously shown to react with an apical structure of P. falciparum ookinetes, also reacts with a discrete apical structure of P. gallinaceum ookinetes. In membrane feeding assays, MAb 1C3 markedly inhibited P. gallinaceum oocyst development in mosquito midguts. MAb 1C3 affinity isolated an approximately 210-kDa
antigen which, under reducing conditions, became a 35-kDa
antigen. This isolated 35-kDa
protein cross-reacted with an antiserum raised against a synthetic
peptide derived from the P. gallinaceum
chitinase active site, PgCHT1, even though MAb 1C3 did not recognize native or recombinant PgCHT1 on Western blot. Therefore, this affinity-purified 35-kDa
antigen appears similar to a previously identified
protein, PgCHT2, a putative second
chitinase of P. gallinaceum.
Epitope mapping indicated MAb 1C3 recognized a region of
PfCHT1 that diverges from a homologous amino acid sequence conserved within sequenced
chitinases of P. berghei, P. yoelii, and P. gallinaceum (PgCHT1). A synthetic
peptide derived from the mapped 1C3
epitope may be useful as a component of a subunit transmission-blocking
vaccine.