Experimental Gram-Negative Bacterial Sepsis: Reevaluation of the Ability of Rough Mutant Antisera to Protect Mice

Abstract

Rabbit antisera to 3 rough Enterobacteriaceae mutants, the Rc chemotype of Escherichia coli J5, the Rd chemotype of Salmonella typhimurium SL 1032, and the Re chemotype of S. minnesota 595, were administered i.v. or i.p. to outbred Swiss albino mice. Control animals were injected concomitantly with normal serum from the same donors obtained prior to immunization. One hour later, challenge was performed i.p. or i.v. with LD95-100 doses of viable E. coli 018, Proteus mirabilis or Klebsiella pneumoniae. Normal pre-immune rabbit sera, lacking detectable antibodies to the specific challenge bacterial strains or to Ra, Rc, Rd, or Re rough mutants of Enterobacteriaceae, exhibited definite abilities to reduce septic mortality when compared with physiologic sterile saline. Analysis of preimmune sera from individual rabbit donors revealed a wide spectrum of protective activity. Post-immune sera against the rough bacterial mutants, possessing high titers of Rc, Rd or Re antibodies, conferred no protection above that afforded by the corresponding preimmune sera. Only antisera to the specific challenge bacterial strain proved more protective than the corresponding pre-immune sera. Normal horse sera from 6 different sources, obtained from healthy animals never immunized with gram-negative bacterial vaccines, all possessed agglutinating antibodies against Ra and Rc (but not Re) chemotypes of Enterobacteriaceae and all provided high levels of protection against i.v. K. pneumoniae sepsis in mice. Naturally occurring specific antibodies, not the anti-rough mutant antibodies, appeared primarily responsible for such protection since protective activity was markedly reduced (> 95%) by absorption with the homologous (K. pneumoniae) but not with a heterologous (P. mirabilis) smooth species and since this loss of protective activity was unaccompanied by any decline in anti-Ra or Rc titers. Antisera to rough gram-negative bacterial mutants apparently does not confer broad spectrum protection to mice against parenteral challenge with smooth Enterobacteriaceae because of the rise in antibody titer to common core antigens.