For at least a decade, researchers have known that normal bacteria in the gut can induce intestinal immune cells to extend tentacle-like structures, known as dendrites, to "capture" antigens, triggering both immediate and long-term immune responses. 

What was less clear was how the bacteria activate this process. Now, a research team led by Osaka University has found that the molecules responsible have been hiding in plain sight. Metabolites are small molecules produced during metabolism, the chemical processes that occur inside all living cells to keep them ticking over.

Metabolic pathways have been intensively studied in many organisms, with most common metabolites having very few secrets. Yet, in a recent paper published in the journal Nature, the team describes how they made an important discovery—two well-known metabolites, pyruvate, and lactate, are in fact the instigators of dendrite protrusion by CX3CR1+ macrophages in the small intestine.

"After studying the available research, we hypothesized that bacterial metabolites present in the small intestine could possibly mediate dendrite protrusion," explains lead author Naoki Morita. "After purifying different fractions from the contents of the small intestines of mice, we discovered that lactic acid and pyruvic acid, produced by lactic acid bacteria in the normal gut flora, act directly on intestinal macrophages."

Next, the researchers identified GPR31, a protein residing on the surface of small intestinal macrophages, as the specific receptor for the two metabolites. Mice lacking GPR31 showed reduced dendrite protrusion by CX3CR1+ cells after being administered pyruvate or lactate and, as a result, decreased antibody production following infection with a non-pathogenic strain of Salmonella. However, the most significant revelation was yet to Come.

"We then examined whether pre-treatment of normal mice with pyruvate or lactate as well as non-pathogenic Salmonella could protect against infection with a virulent strain of the bacterium," says co-lead author Eiji Umemoto. "As we predicted, normal pre-treated mice, but not pre-treated GPR31-defective mice, showed increased survival and an enhanced immune response following infection with the virulent Salmonella strain."

Corresponding author Kiyoshi Takeda explains that the research has multiple clinical applications. "Because these metabolites enhance the immune response, they could be used to improve the effectiveness of oral vaccines, while GPR31 is a promising target for therapies aimed at eliminating intestinal pathogens. Because of this, we expect that lactic acid, pyruvic acid, and GPR31 will all be explored in the near future as new targets for activating immunity."

Cryptosporidium primarily infects the distal small intestine. Immunocompetent hosts control and eliminate the infection, which typically causes acute, self-limited watery diarrhea lasting 5 to 10 days. However, in patients with defects in cellular immune responses (e.g., AIDS, malnutrition, or defects in the CD40-CD154 system), Cryptosporidium frequently causes persistent or chronic diarrhea and may also involve the biliary tract.

critical role in the control of human cryptosporidiosis.

In malnourished children, persistent diarrhea is associated with increased susceptibility to recurrent diarrheal episodes, which can lead to death or chronic nutritional and cognitive sequelae. Thus, the host immune response plays a critical role in the control of human cryptosporidiosis.

Although extensive studies with various animal models have provided important insight into the host immune response towards Cryptosporidium parvum, the ability of these models to explain the human immune response is limited. The clinical picture in rodents differs from that in humans, as mice do not get diarrhea after infection.

Nonhuman primates, although probably the best in vivo model to mimic human disease, are difficult to work with, expensive, and not widely available. Cryptosporidium hominis, the pathogen causing most human cryptosporidiosis, infects only humans and gnotobiotic pigs, thus limiting data from animal models.

Most importantly, the comparison of animal and human data has shown that the immune response towards Cryptosporidium in humans differs significantly from that in animals; for example, in mice gamma interferon (IFN-γ) production seems to be associated with the innate and primary immune responses, whereas in humans it is most probably associated with the memory response towards the parasite. Conducting studies to elucidate human mucosal immune responses is difficult.

Patients with a natural infection would be the ideal subjects to study, but it is difficult to identify cases. Healthy human volunteers can be studied, but they typically experience a milder illness than malnourished children and AIDS patients.

Human intestinal tissue samples can be obtained only by invasive procedures, limiting the numbers of subjects and samples available. Some data can be obtained from in vitro infections, but most of the target cells are immortalized and may not be ideal for studying mechanisms involving apoptosis. Furthermore, the immune cells in the peripheral blood may exhibit properties different from the properties of cells found in the intestinal compartment.

Thus, knowledge about the human immune response towards Cryptosporidium infection is far from complete. Still, important recent advances have been made. The goal of this paper is to review the current literature to provide an understanding of the human immune response towards the parasite. We include some relevant data from other models only when the data shed light on studies performed with human cells or tissues.