PATHOGENESIS OF INFECTIOUS AGENTS 
Gary K. Schoolnik, M.D., Associate Investigator 
Two areas of investigation are being pursued in 
Dr. Schoolnik's laboratory: 1) the pathobiology of 
infections of mucous membranes and 2) the immu- 
nochemistry of small peptides. 
I. Pathobiology of Mucosal Infections. 
Previous studies have focused on the molecular 
mechanisms by which microorganisms bind host 
epithelial cells that line the surfaces of the respira- 
tory, gastrointestinal, and genitourinary tracts. This 
process is mediated in most instances by a protein- 
aceous appendage of the microorganism, termed 
adhesin, and the corresponding epithelial cell re- 
ceptor to which the adhesin binds in a stereoscopi- 
cally specific manner. In the past year two related 
aspects of this binding event have been studied. 
First, surface-exposed adhesin proteins exhibit anti- 
genic diversity, presumably to evade the host im- 
mune response; the genetic and structural basis for 
this phenomenon has been investigated. Second, 
after the adhesin-mediated attachment of the mi- 
crobe to the epithelial cell, penetration of the 
bound cell by invasive microorganisms ensues; this 
process is mediated in some bacteria by a second 
microbial protein, termed invasin. The structure 
and function of one such invasin was studied. 
A. Antigenic diversity of microbial adhesin pro- 
teins. Several bacterial species that are pathogenic 
but taxonomically diverse, including Neisseria 
gonorrhoeae, Pseudomonas aeruginosa, Bacte- 
roides nodosus, Vibrio cholerae, and Moraxella 
bovis, produce speciali2ed adhesin proteins or pili, 
which radiate from the bacterial surface as thin, 
flexible filaments; each filament is composed of 
multiple, identical repeating subunits of —20 kDa. 
The amino acid sequences of the pili subunits of 
the above-noted bacterial species are related but 
not identical: the amino terminus of each is an un- 
usual amino acid, A^-methylphenylalanine, and their 
sequences are nearly homologous for the first 45 
residues. Thereafter their primary structures di- 
verge. Genetic mechanisms for the production of 
antigenic diversity and the extent of the diversity 
have been evaluated for several of these species. 
The genome of F aeruginosa, a species that exists 
most often as a free-living organism in water, con- 
tains a single pili subunit gene; antigenic diversity 
arises as a result of accumulated point mutations. 
Thus this species, which infrequently contends 
with the host immune system, has a limited capac- 
ity for the expression of antigenic diversity. In con- 
trast, A^. gonorrhoeae exists only on human mucous 
membranes, is continually exposed to the selective 
pressures of the immune system, and possesses a 
genetic mechanism for the production of a large 
number of pili antigenic variants. The basis for this 
diversity is the presence of multiple variant pili sub- 
unit gene sequences in the genome of each strain, 
each variant sequence coding for hypervariable re- 
gions of the subunit that specify unique, immu- 
nodominant antigenic determinants. Expression 
of an antigenic variant occurs when the corre- 
sponding pili subunit gene sequence is recombined 
into an expression locus yielding a gene conversion 
event that links sequences coding for the conserved 
amino-terminal domain with sequences coding for 
the hypervariable carboxyl-terminal domain of the 
pili subunit. 
Intermediate with respect to pili antigenic diver- 
sity between F aeruginosa and A^. gonorrhoeae is 
M. bovis, the etiologic agent of bovine keratocon- 
junctivitis. M. bovis exists in nature in three ecolog- 
ical niches: as a pathogen of bovine conjunctival 
membranes, where it is exposed to IgA and IgG in 
tears; as a commensal bacterium of the face fly; and 
as a slowly replicating organism in fomites such as 
dust. Thus in one ecological niche (ocular mem- 
branes) the expression of pili antigenic variants by 
M. bovis may be essential for its survival, whereas in 
another (e.g., dust) it may be irrelevant. In contrast 
to A'; gonorrhoeae, in which a single strain can se- 
quentially express many pili antigen variants, and F 
aeruginosa, in which a single strain expresses only 
one pili antigenic type, the expression of pili by M. 
bovis oscillates between two pili antigenic variants, 
a and P; simultaneous expression of both pili types 
does not occur. 
The complete a and (BM bovis pili amino acid se- 
quences were determined, compared, and found to 
be —70% homologous, indicating that they are the 
products of two separate but related genes. Re- 
gions of sequence heterogeneity were found to 
specify a- or P-specific pili epitopes, since the two 
proteins exhibit only 50% shared antigenicity. 
Pathogenicity studies in calves of a- or (3-pili ex- 
pressing variants of the same strain indicated that (3- 
pili confer or are associated with a relative advan- 
tage during the first stages of ocular infection. The 
Continued 
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