Effects of probiotics on intestinal immunity
OBJECTIVE NUTRITION n°65 (September 2002)
By Marie-Christiane Moreau
INRA, Jouy-en-Josas, France

Of the many functions of the intestine, immune functions are particularly important: they suppress immune responses against dietary proteins, thus eliminating the risk of food allergy or hypersensitivity while making possible the production of immune responses against enteropathogenic microorganisms.
These functions are strongly influenced by the intestinal bacteria, which are part of the resident intestinal microflora or which are provided by certain foods that contain high amounts of microorganisms "probiotics".

It has been known for hundreds of years that fermented milk has a favorable effect on diarrhea.  In the early 1990's, Nobel Prize winner Metchnikov attributed the longevity and exceptionally good health of Bulgarians to the existence of large numbers of lactic acid bacteria in yogurt.
These microorganisms, which may have a favorable effect on our health, are called "probiotics": "Living microorganisms, which, when ingested in appropriate quantities, have beneficial effects on the host organism's health by improving the host's intestinal microbial balance." Currently, different products containing probiotic lactic bacteria have been developed, particularly yogurt and other types of fermented milk that contain a particular strain of probiotic, either alone or in combination with lactic starters in yogurt (insert 1).  For the moment, this approach only involves fermented milk since microorganisms present in cheeses have not yet been the subject of such research and development.


Insert 1

The intestine: an unknown organ

The intestine is the body's most important immune function-related organ: approximately 60 % of the body's immune cells are present in the intestinal mucosa (figure 1).

Figure 1

The intestine also contains an extensive microflora – 100,000 billion bacteria located mainly in the colon.  They are distributed into dominant groups (active) and subdominant groups according to a microbial balance that, on the whole, is specific for each animal species and period of life.
The particular characteristics of the immune system associated with the intestine is of being in constant contact with dietary proteins and those of bacteria in our intestinal flora.  The immune system prevents immune responses against dietary proteins (prevention of food allergies) and against bacteria (prevention of chronic inflammatory diseases of the intestine).  This important function, the "oral tolerance", makes it possible to eat and to tolerate the resident intestinal microflora. The immune system associated with the intestine must also protect the host against the penetration of resident bacteria into the systemic compartment (translocation) and against invasion by pathogenic microorganisms, viruses (rotavirus, polio virus), bacteria (salmonella, listeria, clostridium, etc.) and parasites (toxoplasma), many of which cause diarrhea.  Consequently it develops protective immune responses, including the synthesis of antibodies corresponding to a specific class of immunoglobulins, the secretory IgA, adapted to the intestine, and whose functions are to prevent a bacterial translocation, to neutralize toxins, to inhibit viral cell multiplication in the enterocyte, and to block adherence of bacteria to the mucosa.

Microorganisms and intestinal immunity

Comparative studies between germ-free mice (bred in sterile compartments away from all bacterial contamination) and "standard" mice (bred traditionally in animal houses), demonstrated the strong influence played by the resident intestinal microflora on the maturation and development of the intestinal immune system and, consequently, on its fonction.
Whenever there is an imbalance in the resident intestinal flora (ingestion of antibiotics, disease, diet, stress, laxatives, etc.), the functioning of the immune intestinal system will not be optimum.  In children aged 0 to 2 years, the intestinal flora, which is not yet diversified and thus fragile (about 10 bacterial strains, versus 200 to 400 in adults), is readily impaired and subject to imbalances.  Therefore, a child must acquire as quickly as possible a functional intestinal immune system in order to prevent development of food allergies and to fight against certain enteropathogenic microorganisms to which the child is very susceptible, such as the rotaviruses.
Living microorganisms ingested with some foods may have beneficial effects by compensating for the deficiencies of an imbalanced resident intestinal microflora. Contrary to resident bacteria, probiotics do not durably colonize the gastrointestinal system.  They only transit through, but may exert effects on the host during this time of passage with some conditions presence of large quantity in the food (at least 10⁸/g) and possess immunomodulating properties equivalent to those of resident intestinal bacteria. They must resist gastric acidity and biliar salts to survive in sufficient numbers in the digestive tract.  In spite of this "travel" stress, it has recently been shown that such bacteria still demonstrate metabolic activity.

Clinical data in humans

The most conclusive clinical studies involve children in whom the curative and preventative effects of probiotics have been demonstrated in rotavirus-associated diarrhea including the demonstration of a correlation between the decrease in diarrhea duration and the stimulation of anti-rotavirus IgA antibody synthesis (insert 2). 

Insert 2

Regarding food allergies, a recent Finnish study has demonstrated for the first time a preventive effect produced by ingestion of the L.rhamnosus GG strain on atopic eczema development in infants from high-risk families.  In a group of 134 mothers breastfeeding their babies, half received 1010 L. rhamnosus per day for one month prior to delivery and then during the next six months (mother and/or baby), and the other half received a placebo.  At two years, the incidence of atopic eczema was 23 % in treated children versus 46 % in the placebo group. Immunity mechanisms underlying this protection are not yet understood.
At approximately 2 years of age, the composition of a child's intestinal microflora is close to that of an adult's.  At the same time, the above-mentioned disorders disappear in the majority of cases.

In adults, the stimulation of the immune system by probiotics is much more difficult to demonstrate.  A "leads to follow" approach is preferred over that of "near certainty".  Complex, costly studies using different probiotics and conducted on cohorts of subjects who are often not comparable, sometimes yield contradictory results.  Several studies have shown a significant preventive effect resulting from ingestion of different probiotics against antibiotic-associated diarrhea.  The role played by immunity is not certain, since this protection may also be accounted for by the antagonistic effects of probiotics against opportunistic pathogenic microorganisms.  The origin of traveler's diarrhea remains poorly investigated and may be of a different origin.  Prevention attempts using probiotics have yielded contradictory results.

In the treatment of vaginal candidiasis, a beneficial effect of probiotics seems to exist, even though the underlying mechanism involved is unknown.  Experimental animal studies demonstrated preventive and curative effects of different strains of bifidobacteria  against colon cancer but such demonstration continues to be vague in humans.  However, a protective effect against the recurrence of superficial bladder cancer has been demonstrated using the L. casei Shirota strain. The mechanisms may involve an inhibitory effect of probiotics on the synthesis of carcinogenic metabolites by the resident intestinal microflora, with the possible intervention of stimulation of NK immune cells (with an anti-tumor activity).  Lastly, in the treatment of some gastrointestinal inflammatory disorders in which an abnormal immune response was directed against bacteria in the intestinal microflora, the ingestion of probiotics has yielded contradictory results.  A recent, encouraging study using a combination of eight strains of lactic acid bacteria has demonstrated an increase in the duration of remission of acute inflammatory episodes in the treated group.

Although no direct action on the intestinal immune system is involved, several studies have reported an effect of different probiotic strains on the stimulation of phagocytic activity of circulating immune cell populations which participate in so-called "innate" immunity.  This type of immunity allows the body to quickly eliminate opportunistic microorganisms before subsequent activation of immune responses specific to "acquired" immunity (antibodies, cellular responses).  Furthermore, phagocytic cells play a primary role in acquired immunity: presentation of the antigen, synthesis of regulatory molecules of immune responses called cytokines.  Thus their activity is very important for the proper functioning of the immune system.

Long-term effects?

The probiotic function in the long-term prevention of diseases in humans is unknown.  Similarly, few interesting studies conducted in healthy subjects have been published. On the one hand, they sometimes describe no effect thus supporting the hypothesis that the action of probiotics may only be to optimize immune function when the resident bacterial microflora failed to perform and not to "over stimulate" them needlessly.  On the other hand, their objective is not to study the disorders themselves, but rather the probiotic-related effects on immune cells in order to better understand the causative mechanisms involved.  Thus, stimulation of phagocytosis, synthesis of cytokines, secretory IgAs, cytotokic activity of NK cells, etc., are so many factors that make it possible to understand how probiotics exert their immunomodulator activity.
Studies have only just begun in the elderly, in whom an alteration in the intestinal microflora balance is often observed at the same time as the decline in immune functions.

Conclusion

A great deal of research is still necessary to better understand how and under what conditions probiotics act, particularly in healthy subjects.  Their effects will be all the more effective when the intestinal microflora, fragile or altered, is not or is no longer able to fully produce its stimulant action on the immune system.  From a basic standpoint, knowledge of the bacteria immunomodulatory function of bacteria from the resident intestinal microflora and the mechanisms involved is essential to better understand the effect of probiotics.  It is associated with that of immunomodulatory properties specific to each probiotic strain.  All strains that belong to a given bacterial genus do not have the same immunomodulatory potential.  It is unlikely that a single strain may stimulate all aspects of the immune response!  More targeted studies may then be considered depending on the subject's age, treatment of a disorder, or long-term prevention.

Marie-Christiane Moreau
INRA, Jouy-en-Josas, France

REFERENCES

- Fuller R. 1989. A review : Probiotics in man and animals. J Appl. Bacteriol., 66, 365-378.

- Moreau MC and Gaboriau-Routhiau V. 2000 Influence of resident intestinal microflora on the development and functions of the intestinal-associated lymphoid tissue, in Probiotics, eds Fuller and Perdigon, Kluwer academic publishers; vol 3, 69-114 and in Microb Eco Health Dis. 2001. 13, 65-86.

- Kalliomaki M., Salminen S., Arvilommi H., Kero P., Koskinen P and E. Isolauri. 2001. Probiotics in primary prevention of atopic disease : a randomised placebo-controlled trial. Lancet, 357, 1076-1079.

- Marteau P.R. De Vrese M., Cellier C.J., and J. Schrezenmeir. 2001. Protection from gastrointestinal diseases with the use of probiotics. Am. J. Clin. Nutr., 73 (Suppl) 430S-436S.