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Consistent anti-probiotic IgA activity in probiotic treated and not treated subjects suggests careful evaluation using such bacteria as probiotics

Copious research data from trials in vitro and on experimental animals are available showing that the gut immune system could be down- or up- regulated in order to improve the overall health of the host. In some cases it is claimed that humoral immunity was increased, with special emphasis on the stimulation of specific response against certain pathogens. However the development of specific immune response towards commensal bacteria and probiotics is less documented.

Indeed a prolonged secretion of immune globulins is a cost for the host and could also reduce the chance of survival of the probiotic in the gut. Changing levels of Lactobacillus-species specific IgA were observed in mice mono-associated with 2 lactobacilli showing similar in vitro adhesion patterns (L. johnsonii NCC 533 or L. paracasei NCC 2461). Nevertheless the variations of Lactobacillus-species specific IgA are in general not studied in normally fed producing animals.

Data refer to blood and saliva samples collected at different times during two different trails in witch L.rhamnosus GG (Lb-R) (Trial A) and strain 001T of the new species Lactobacillus amylovorus-like strain 001 (Lb-S) (Trial B), were respectively supplemented to pigs. The first microbe is a probiotic approved for human use and is considered to be not commensal of pig, while the second was isolated in piglets fed fermentable diets (collaborative work).

In both trials, pigs, weaned at 21 days of age, were fed control diet or control diet plus 10x10 CFU probiotic/day. Pigs were orally challenged with 10x10 CFU E. coli K88ac O148 (F4) on day 7, and sacrificed after another week.We wanted to assess at different times after weaning the IgA activity against a pig-specific and a pig-not specific strain. Secondly we wanted to verify if part of this IgA strain-specific activity was partially related to cross-reactivity between two different Lactobacillus-species.

Seric IgA anti-L.rhamnosus GG were present at all the samplings, even before the dietary supplementation with L.rhamnosus GG and in the control group. This contrast with the fact that L.rhamnosus GG is not considered a strain typical of the pig. Indeed, cecum samples obtained from control subjects were negative for the presence of DNA from L.rhamnosus GG (at the contrary, most of the probiotic-fed pigs were positive). A similar observation was done in the second trial, where seric IgA anti L.amylovorus-like strain 001 were also detected in control pigs that were negative in ileum for the presence of this particular strain.

All these observations can be explained by the presence of a cross reactivity between different bacteria. If the values of probiotic specific IgA are expressed on total IgA content and as a difference between different times of samplings, we can see that the strain-specific IgA relative content in the Lactobacillus group increased between start and 1st week; this was not observed after and for the control group. This observation shows that there was an additional short-term specific immune response in the probiotic group, in agreement with observations on germ-free piglets fed with non-pathogenic E.coli.

Cross reactivity index evaluated by pre-absorption test for different samples (blood or saliva) and for different specific IgA’s was high. There is a consistent debate in the literature about the presence of antibodies that apparently react with many different microbes. One possible explanation is the similarity of some amino acid and sugar motifs in the structure of the S-layer that protects Gram+ cells; a second hypothesis is that part of these IgA's are "polyreactive" or "natural". Then it is not yet clear the functional significance of the presence of IgA against commensals: it could constrain these bacteria inside the gut lumen, but it could also improve their chance of adhesion to mucous and their persistence in the gut. In any case, when probiotic are supplied to weaning pigs, the possible action of already present secretory IgA should be considered. This could also explain unfavourable results of the probiotic strategy.

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