Immunology of Cholera

Experimental immunisation and protection of guinea pigs with Vibrio cholerae toxoid and mucinases, neuraminidase and proteinase

As measured by fluid accumulation in ileal loops, Vibrio cholerae mucinase complex, with or without toxoid, protected guinea pigs from challenges with V. cholerae live organisms and enterotoxin. The neuraminidase and proteinases of the complex were combined in modified oil emulsion or aluminum hydroxide adjuvants and the resultant vaccines given by the parenteral or oral routes. There was little difference between the two types of adjuvant. Control of stomach acidity improved oral vaccination. Animals injected intramuscularly (i.m.) with toxoid-containing vaccines were protected from challenge with cholera toxin (CT) whereas those given oral doses were not. Toxoid plus killed V. cholerae cells elicited a more effective protection against toxin challenge than killed V. cholerae cells alone. Vaccines containing mucinases, with or without toxoid, protected the animals from a live V. cholerae challenge. The anti-mucinase immune response may prevent adhesion of the V. cholerae cells and hence reduce delivery of toxin to receptors. These mucinases, neuraminidase and proteinases, may be useful components of acellular, toxoided cholera vaccines for human immunisation.

Homologous sequences in cholera toxin A and B subunits to peptide domains in myelin basic protein

Recent reports that myelin basic protein (MBP) can be ADP-ribosylated and contains specific sites that bind GTP and GM1 ganglioside, have suggested an analogy to the properties of cholera toxin. Comparisons of pairs of sequences between these two proteins yielded two regions of homology between MBP and the cholera toxin B (chol B) subunit, and one region of homology with the cholera toxin A (chol A) subunit. The matching sites within chol B consisted of a 17 amino acid residue sequence (residues 30-46 in chol B and residues 102-118 in human-MBP, hMBP, p less than 0.0007) and an 11 residue span (residues 31-41 in chol B and sequence 29-39 in hMBP, p less than 0.0004). The homologous site within chol A corresponded to an 11 residue span (residues 130-140 in chol A and 67-77 in hMBP sequence, p less than 0.00007). Since portions of the cholera toxin sequence are virtually identical to sections of the sequence in E. coli toxin, the homology is also valid for the same sequences in this toxin. The highly antigenic behavior of MBP that is related to the induction of experimental allergic encephalomyelitis may be paralleled by comparable neural pathology from the homologous regions of cholera toxin.

Immunoglobulins in bile and serum of the rabbit associated with protection after Vibrio cholerae infection and vaccination

Previous studies have shown that cholera, as well as protective immunity against infection with Vibrio cholerae, can be induced in the rabbit. This protection is long-lasting (up to 30 months) and is characterized on challenge by rapid, symptom-free disappearance of V. cholerae from the intestine; we therefore believe this to be vibriocidal protection. In this study, we analysed the humoral and secretory immune response against various subcellular V. cholerae components in vibriocidally protected, non-vibriocidally protected, and unprotected animals. Only vibriocidal protection was found to be associated with high levels of biliary IgA directed against lipopolysaccharide O antigen. We did not find such a correlation between either type of protection and response in serum. Therefore, anti-lipopolysaccharide antibodies are essential in protection against experimental infection with V. cholerae.

Vibrio cholerae infection and acquired immunity in an adult rabbit model

We modified the rabbit model for enteric infection by Vibrio cholerae developed by Spira et al. and designated the RITARD (for removable intestinal tie-adult rabbit diarrhea) model (20). Our modification DISC comprises a permanent ligation of the cecum (C) to prevent resorption of the fluid secreted by the small intestine, a temporary ligation of the small intestine (S) to enable the bacteria to colonize, and duodenal inoculation (DI) of the challenge material. The main difference between RITARD and DISC is that in the latter model the challenge material is injected into the duodenum approximately 10 cm distal to the stomach instead of into the jejunum. Four out of 5 V. cholerae strains tested, including 2 serotypes and 2 biotypes, were able to elicit a massive and usually fatal cholera-like diarrhea. The virulence depended strongly on the culturing conditions. One strain, C5, caused fatal diarrhea in a dose of about 1000 organisms, even if the temporary ligation was omitted (DIC model). Other modifications were the DIS and the DI model in which the permanent ligature of the cecum or both ligatures were omitted. Duodenal inoculation of organisms in a dose of 100 X the minimum infective dose (MID) in the DIS or DI model did not cause any disease symptom. However, such inoculations were found to cause protection against subsequent challenges with 100 X MID of homologous and heterologous organisms up to 52 weeks after duodenal inoculation. Subcutaneous injection with classical, whole cell cholera vaccine gave only partial protection of short duration. This model might contribute to the understanding of the pathogenesis of cholera as well as to the improvement of efficacy testing of cholera vaccines.

Evaluation in humans of attenuated Vibrio cholerae El Tor Ogawa strain Texas Star-SR as a live oral vaccine

Texas Star-SR, an A- B+ mutant derived by nitrosoguanidine treatment from Vibrio cholerae El Tor Ogawa strain 3083, was fed to 68 volunteers as an oral vaccine in doses of 10(5) to 5 X 10(10) organisms with NaHCO3. Sixteen (24%) vaccinees experienced some loose stools (unrelated to vaccine dose), but in only one did the total stool volume exceed 1.0 liter. The vaccine strain was cultured from duodenal fluid of 35 of 46 (76%) persons who ingested doses of 10(8) organisms or greater. No A+ B+ toxinogenic revertants were found among 456 clinical isolates tested. Sixty-three vaccinees (93%) manifested seroconversions of vibriocidal antibody, whereas only 20 (29%) had significant rises in serum antitoxin titers. Paired intestinal fluids from 41 volunteers showed significant rises of secretory immunoglobulin A against lipopolysaccharide (29%), Ogawa outer membrane preparation (29%), and toxin (12%) antigens. In challenge studies with pathogenic V. cholerae El Tor Ogawa and El Tor Inaba, the attack rate in vaccinees (7 of 25) was significantly lower than in controls (18 of 25) (vaccine efficacy, 61%); furthermore, the diarrheal stool volume in vaccinees was significantly less than that in controls (P less than 0.01). Texas Star-SR served as a prototype to investigate the concept of immunoprophylaxis by means of attenuated strains as oral vaccines. These observations provide an invaluable background for planning future studies with newly developed attenuated strains prepared by recombinant DNA techniques.

Rabbit leukocyte surface antigens defined by monoclonal antibodies

Several monoclonal antibodies (mAb) against rabbit leukocytes were characterized in binding and functional studies. mAb 1.24 stains thymocytes, bone marrow cells, peripheral T and B cells and blood monocytes. T cells express more 1.24 antigen than B cells. In the absence of added complement (C), mAb 1.24 inhibits alloantigen-, concanavalin A (Con A)-, and phytohemagglutinin (PHA)-, but not pokeweed mitogen (PWM)- or anti-immunoglobulin (Ig)-induced cell proliferation. It also strongly blocks anti-sheep erythrocyte plaque-forming cell responses. A second mAb, designated 4.B9, binds to 20% of thymocytes and to most, if not all, peripheral T cells and in vitro-activated T cell blasts. A third one, 10.B3, is reactive with the nearly entire thymocyte and a major peripheral T cell population. Two-color membrane immunofluorescence reveals the presence of a small population of peripheral blood leukocytes which bear surface Ig and are weakly stained by mAb 4.B9 and 10.B3. Without C, both 4.B9 and 10.B3 inhibit Con A- and PHA-induced mitogenesis, but have no effect on PWM-, antigen-, or alloantigen-induced cell proliferation. Depletion of 4.B9+ cells by panning or complement lysis completely abrogates proliferative responsiveness to antigen and alloantigen, significantly reduces responsiveness to the T cell mitogens Con A and PHA, but enhances that to the B cell mitogen anti-Ig. A fourth mAb, 12.C7, binds to 60% of thymocytes and to 10-30% of peripheral T lymphocytes at high-level fluorescence. T cell blasts obtained in mixed leukocyte reactions are partially stained by mAb 12.C7, while those obtained after Con A or PHA activation are not. In addition, mAb 12.C7 is completely unreactive with B cells or monocytes. Without complement, it does not seem to interfere with any of the in vitro functions tested. All antigens studied here do not appear to be expressed in nonleukon tissues, as they do not bind to erythrocytes and are absent from brain, heart, liver and kidney as shown by quantitative absorption analysis.

Mechanism of action of choleragen

Choleragen exerts its effect on cells through activation of adenylate cyclase. Choleragen initially interacts with cells through binding of the B subunit of the toxin to the ganglioside GM1 on the cell surface. Subsequent events are less clear. Patching or capping of toxin on the cell surface may be an obligatory step in choleragen action. Studies in cell-free systems have demonstrated that activation of adenylate cyclase by choleragen requires NAD. In addition to NAD, requirements have been observed for ATP, GTP, and calcium-dependent regulatory protein. GTP also is required for the expression of choleragen-activated adenylate cyclase. In preparations from turkey erythrocytes, choleragen appears to inhibit an isoproterenol-stimulated GTPase. It has been postulated that by decreasing the activity of a specific GTPase, choleragen would stabilize a GTP-adenylate cyclase complex and maintain the cyclase in an activated state. Although the holotoxin is most effective in intact cells, with the A subunit having 1/20th of its activity and the B subunit (choleragenoid) being inactive, in cell-free systems the A subunit, specifically the A1 fragment, is required for adenylate cyclase activation. The B protomer is inactive. Choleragen, the A subunit, or A1 fragment under suitable conditions hydrolyzes NAD to ADP-ribose and nicotinamide (NAD glycohydrolase activity) and catalyzes the transfer of the ADP-ribose moiety of NAD to the guandino group of arginine (ADP-ribosyltransferase activity). The NAD glycohydrolase activity is similar to that exhibited by other NAD-dependent bacterial toxins (diphtheria toxin, Pseudomonas exotoxin A), which act by catalyzing the ADP-ribosylation of a specific acceptor protein. If the ADP-ribosylation of arginine is a model for the reaction catalyzed by choleragen in vivo, then arginine is presumably an analog of the amino acid which is ADP-ribosylated in the acceptor protein. It is postulated that choleragen exerts its effects on cells through the NAD-dependent ADP-ribosylation of an arginine or similar amino acid in either the cyclase itself or a regulatory protein of the cyclase system.

Immunological properties of Vibrio cholerae lipopolysaccharides

Immunological effects of wall lipopolysaccharide (LPS) preparations obtained from Vibrio cholerae Inaba 569B, Ogawa NIH 41 and NAG 4715 strains by the hot phenol-water procedure were examined in mice. Although these LPS lack KDO, which are basic components of the core region of most gram-negative LPS, they still have potencies as B-cell mitogens, adjuvants, immunosuppressants, polyclonal B-cell activators and phagocytic stimulants for macrophages. The activities of these V. cholerae LPS on murine immune system seemed to be weaker than those of Salmonella typhimurium LT2-LPS. Among these V. cholerae LPS, NAG 4715-LPS showed the strongest mitogenic activity and phagocytic stimulation, while the potencies of this NAG 4715-LPS for the induction of polyclonal B cell activation, adjuvant effects and immunosuppression did not seem to be greater to those of the other LPS.