Nontypable Haemophilus influenzae (NTHi) are primarily noninvasive human respiratory tract pathogens.  NTHi can reside in the respiratory tract as a commensal or give rise to local infections, including otitis media, bronchitis, sinusitis, and rarely, pneumonia (Bluestone, C. D., and J. O. Klein, In Pediatric Otolaryngology.,  356 (1983); Bluestone and Stool ed.  W. B. Saunders Co. Philadelphia.;  Musher, D. M. et al.,  Ann.  Intern.  Med.  99:344-350 (1983)).  Several potential adherence factors have been described for Haemophilus influenzae (both typable and nontypable) adherence to human cells, including four classes of fimbriae/pili and two high molecular weight proteins with similarity to the filamentous hemagglutinin of Bordetella pertussis (St. Geme, J. W., et al.,  Proc.  Natl.  Acad.  Sci.  USA 90:2875-2879 (1993)).  Pili are bacterial surface antigens.  They are protein appendages consisting of a helically symmetrical assembly of major protein (pilin) subunits.  Some pili can also carry from two to three minor proteins assembled on their tips.  One of these proteins, adhesin, carries the active site for pilus adhesion to specific membrane receptors on human and animal cells. 
One class of pili/fimbriae has been widely studied, the long thick positive (LKP) family.  LKP pili are expressed by both typable and nontypable H. influenzae (Hib).  The pili in this family have a characteristic morphology, partially shared adhesion specificity and their structural proteins share amino acid sequences.  These pili are hemagglutination positive and mediate attachment to human mucosal cells (Brinton, C. C. et al.,  Pediatr.  Infect.  Dis.  J. 8 Suppl.:54-61 (1989)).  Hemagglutination of human erythrocytes is accomplished via binding to the AnWj blood group antigen while binding to epithelial cells involves a sialic acid containing lactosylceramide receptor (van Alphen, L. et al.,  Infect.  Immun.  69:4473-4477 (1991)). 
The LKP family has been divided into different strain specific serotypes based on reactivity to polyclonal antisera raised against the purified pili.  Little cross reactivity among pili serotypes has been observed (Brinton, C. C., et al.,  Pediatr.  Infect.  Dis.  J. 8 Suppl.:54-61 (1989)). 
Inhibiting, or blocking, LKP pilus-mediated adhesion by H. influenzae to cells can prevent H. influenzae diseases.  Purified, intact LKP pili have been shown to be vaccine candidates for NTHi otitis media in the chinchilla model, conferring protection against challenge with NTHi strains bearing homologous pili serotype (Karasic, R. et al.,  Pediatr.  Infect.  Dis.  J. 8 (Suppl.):  S62-65 (1988)).  However, because protection is pilus-specific, for broad protection, a vaccine would be required to be multivalent, including the most frequently occurring serotypes of pili in the natural population of pathogens.  LKP pilin structural genes have been cloned and sequenced by several groups (Coleman, T. et al.,  Infect.  Immun.  59:1716-1722 (1991); Forney, L. J. et al.,  Infect.  Immun.  59:1991-1996 (1991); Kar, S., et al.  Infect.  Immun.  58:903-908 (1990); van Ham, S. M., et al.,  EMBO Jour.  8:3535-3540 (1989)), but only the genes responsible for pili serotypes 1 and 4 have been identified.