surface polyphosphate than other Neisseria, but the function and immunogenic potential of this surface polyphosphate are unknown.
Gonococcal pili (fimbriae) are important in mediating attachment to specific mucosal cells and in resisting ingestion by neutrophils. Polyclonal anti-pilus antibodies raised to whole purified pili block adherence of the homologous (but not heterologous) strain to fallopian tube mucosa (McGee, personal communication, 1984) and erythrocytes (Buchanan and Pearce, 1976), and are opsonic (Siegel et al., 1982). Polyclonal antipilus antibodies also block attachment to human buccal, vaginal, and cervical cells (Tramont, 1977). Unfortunately, gonococcal pili are antigenically heterogeneous in different strains, and in different isolates of a single strain. Brinton found no more than 25 percent shared pilus antigens among 49 of 50 tested clinical isolates (Brinton et al., 1978). Because pilus vaccines have been reported to be effective for certain enteric infections (e.g., E. coli diarrhea in piglets; Brinton et al., 1983), there has been much interest in pili as a gonococcal vaccine candidate. Recent work on pilus vaccines for gonorrhea has been described by Brinton (1984) and Tramont (1984).
Understanding the possible role of pilus antigens in a vaccine requires a more detailed discussion of pilus structure and function. The pilus is made up of repeating units of an approximately 19 Kd subunit. The hydrophilic C-terminal domain (CNBr fragment 3) is chemically and antigenically highly variable, whereas the hydrophobic N-terminal domain (CNBr fragment 1) is common on the small number of stains tested thus far (see Figure L.1) (Schoolnik et al., 1983). The variable C-terminal end is immunodominant: immunization with whole pili results in antibodies directed almost entirely against CNBr3.
Recent studies by several groups have revealed an immunorecessive pilus domain in the middle of the molecule (CNBr fragment 2) that is shared by all those gonococcal pili tested, and that appears to be involved in attachment to red blood cells. Limited data suggest that immunization of animals with isolated CNBr fragment 2 results in antibody that will recognize this common peptide, reducing attachment to red blood cells by all gonococci (Schoolnik et al., 1983). Immunization with whole gonococci or whole pili does not result in antibodies against CNBr2, presumably because CNBr2 is buried within the three-dimensional configuration of the pilus. If a common pilus structure is required to bind to specific mucosal receptors, it clearly would be advantageous for the gonococcus to be able to “hide” this structure, so infection would generate antibodies only against the external variable domain. If this work is substantiated by future efforts, and if the pilus receptor is the same on red blood cells and mucosal cells, it may be possible to utilize CNBr fragment 2 or a related peptide as a broadly effective vaccine.