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Many pathogenic bacterial species are classified into sero- logical types, distinguished by their carbohydrate-rich surface antigens. These antigens are referred to as serotypic epitopes or glycoepitopes. Glycoepitopes are most commonly associated with capsular polysaccharides (CPS) or the O-antigen domains of bacterial lipopolysaccharides (LPS). The association of specific bacterial serotypes of a given genus with defined clinical infections is a common phenome- non that applies to a variety of gram-negative bacteria and gram-positive bacteria. As discussed below, of the 77 distinct capsular serotypes in the species Klebsiella pneumoniae, only 25 account for most pulmonary and blood infections (12). Like- wise, of the more than 90 capsular serotypes of Streptococcus pneumoniae, only 23 account for most infections, and these are included in the current capsular vaccine (25). The surfaces of the pulmonary pathogens Klebsiella pneu- moniae and Streptococcus pneumoniae are coated with a variety of glycoconjugates that confer specific properties. For exam- ple, the polysaccharide capsules enable these pathogens to resist phagocytosis. In a few cases, it has been argued that serotype-associated differences in capsular size contribute to virulence and the high frequency of isolation. However, there is no satisfactory explanation why only a limited number of capsular serotypes are responsible for most infections (12, 25). Because lectins are important components of the lung's in- nate immune system, it is reasonable to hypothesize that dif- ferences in the frequencies of isolation of specific serotypes in the setting of pneumonia are in part determined by the recog- nition, or the lack of recognition, of their corresponding gly- coepitopes by lectins. We propose that the lack of recognition of glycoconjugates by one or more lectins of the lung contrib- utes to the high frequency of isolation from pneumonia pa- tients of the pulmonary pathogens bearing reactive glyco- epitopes. Conversely, lectins that recognize serotype-specific glycoepitopes contribute to the eradication of these serotypes, resulting in a lower frequency of their isolation from clinical samples. In this review, we first correlate the sugar specificities of several types of lung lectins with the glycoconjugates on the surfaces of selected pulmonary pathogens. We then briefly describe in vitro data that suggest mechanisms through which the lung C-type lectins contribute to the elimination of specific glycoepitope-bearing serotypes. Finally, we review available evidence supporting our hypothesis that the high frequency of isolation of serotypes bearing a specific glycoepitope results from the ability of these serotypes to evade recognition by lectins of the innate immune system.

infection and immunity

Recognition of Bacterial Surface Polysaccharides by Lectins of the Innate Immune System and Its Contribution to Defense against Infection: the Case of Pulmonary Pathogens