Microbial Pathogenesis 1986,l: 361-371 Strong biotype and serotype cross-protective antibacterial and antitoxic immunity in rabbits after cholera infection Nils Lycke,” A.-M. Svennerholm and J. Holmgren Department of Medical Microbiology, University of GCjteborg, S-413 46 Geteborg, Sweden (Received November 18,1985; accepted in revised form December 13,1985) Lycke, N. (Dept of Medical Microbiology, University of Goteborg, S-413 46 Goteborg, Sweden), A.-M. Svennerholm and J. Holmgren. Strong biotype and serotype cross-protective antibacterial and antitoxic immunity in rabbits after cholera infection. Microbial Pathogenesis 1986; 1: 361-371. We studied whether immunity evoked by infection with classical or El Tor V. cholerae 01 organisms in rabbits (the RITARD model) also gives protection against cholera caused by V. cholerae of heterologous biotype as well as serotype, and whether such protection is antibacterial and/or antitoxic. A primary infection with a classical Ogawa or El Tor lnaba strain resulted in intestinal colonization and diarrhea1 disease in a dose-related manner though the El Tor strain was more virulent. As few as 1 O3 El Tor organisms gave disease in more than 90% of the rabbits as compared to lo9 classical organisms (ED,,); the El Tor strain also gave rise to diarrhea with earlier onset and of greater severity and longer duration. The primary infection induced strong protective immunity against later challenge with either the homologous or the heterologous strain in doses that corresponded to 1000xED9,,. Protection was associated with marked inhibition of colonization, and when rabbits convalescing from cholera infection were challenged with graded doses of bacteria or purified toxin in ligated intestinal loops significant antibacterial as well as antitoxic immunity was evident. Titer rises in serum vibriocidal and anti-lipo- polysaccharide antibodies were similar after infection with either strain, whilst antitoxin titer rises were more marked after El Tor infection. During infection V. cholerae 01 organisms seem to express protective antigens that stimulate immunity which extends across both biotype and serotype barriers. Key words: cholera; Vibrio cholerae; enterotoxins; immunity; RITARD model. Introduction It is known both from epidemiological studies of cholera in endemic areas’-3 and from experiments in human volunteers 4.5 that natural infection with Vibrio cholerae associated with disease gives rise to long-lasting protective immunity against a second attack of cholera. Repeated inapparent exposure to V. cholerae 01 bacteria or organisms expressing cross-reacting antigens may also induce increased resistance to cholera, as suggested by the inverse correlation found between cholera attack rate and age or vibriocidal antibody levels in endemic areas.’ The acquisition of immunity against cholera is thought to depend on the stimulation of the,mucosal immune system of the gut. c In line with this, significant titer rises in locally produced antibacterial and antitoxic antibodies have been found in cholera patients recovering from disease.7,8 *Address correspondence to: Dr Nils Lycke, Department of Medical Microbiology, Guldhedsgatan 10, S- 413 46 Gijteborg, Sweden. Telphone: 00946-31-602070. 0882-401 O/86/040361 +l 1 $03.00/O @ 1986 Academic Press Inc. (London) Ltd