RELATIONSHIPS AMONG CAPSULAR STRUCTURE, PHAGOCYTOSIS, AND MOUSE VIRULENCE IN KLEBSIELLA-PNEUMONIAE

Klebsiella pneumoniae strains of the K2 capsular serotype are usually highly virulent in mice, which is in contrast to the low virulence of most other serotypes. Here we used a genetic approach to examine the r elative contribution of capsule type to the virulence of K. pneumoniae in mice. We used wild-type strains expressing capsular polysaccharide ICPS) serotypes K2 (strain KPA1) and E21a (strains KPB1 and KPC1), wh ich were then used to construct capsule-switched derivatives. The clos e proximity of the cps gene cluster to selectable his markers made it possible to mobilize the cps genes by conjugation from one serotype (d onor) to another (recipient) and to obtain recombinants in which inter serotype switching had occurred by reciprocal recombination. Each caps ule-switched derivative examined of the RPA and KPC strain backgrounds produced a CPS that was immunologically and structurally identical to that of the donor. Strain background was confirmed by demonstrating r estriction fragment length polymorphism patterns identical to those of the respective recipients. The parent strains were then compared with capsule-switched recombinants for phenotypic properties associated wi th virulence. Clearance from the bloodstreams of mice was rapid in ser otype K21a strains of either wild-type or recombinant origin, whereas K2 strains remained viable in the blood during the period examined. Th ese differences appeared to be dependent upon the CPS type but indepen dent of strain background. Binding to macrophages was higher in K21a s trains than in those with the K2 capsule and was also independent of t he strain background. Both blood clearance and macrophage-binding acti vities were completely inhibited by yeast mannan, suggesting that they were mediated via the macrophage mannose receptor. The K2 parent stra in was highly virulent to mice (50% lethal dose [LD(50)], 3 x 10(3)), while the K21a parent strains demonstrated low virulence (LD(50), >2 x 10(8)). Interestingly, the virulence of recombinant KPC10(cpsK2), ori ginally of the KPCl(cpsK21a) background, was intermediate (LD(50), 4 x 10(5)). In contrast, both cpsK21a recombinants of the originally viru lent KPA1(cpsK2) background became nearly avirulent (LD(50), >2 x 10(8 )). Six additional serotypes (K12, K24, K32, K55, K62, and K67) were e xamined, and all showed a positive correlation between the ability of the Klebsiella serotype to interact with a human mannose receptor, as expressed by Cos I cell recombinants, and the LD(50) of the serotype. These results suggest that expression of a capsule which is recognized by the mannose receptor markedly affects the interaction with macroph ages and blood clearance. The virulence of the cpsK2 recombinant of th e KPC background may have been enhanced because it was expressing a he terologous capsule not recognized by the mannose receptor. Thus, this study shows that the capsule type plays an important role in the rate of blood clearance and phagocytosis but contributes only partially to the virulence of K. pneumoniae in mice.