Learning Through Your Gut

What is all the hype about probiotic yogurt these days and why would a person want to have, let alone add, bacteria into their body? The fact is that not all bacteria are bad; some can even be beneficial to our health. The role of bacteria found in the gut include helping with the digestion of food, providing vitamin K, helping protect the colon from the invasion of harmful bacteria, and helping to educate the immune system to differentiate between harmless and harmful bacteria.¹ However, it is still not well understood exactly why the body fails to trigger an immune response against these bacteria.

Our bodies fight invasion through the lymphocytes. These white blood cells are varied and one type, the T cell, produces a response against invasion by activating other cells called regulatory T cells. Lathrop and his associates set out to determine how these cells are involved in regulating immune responses in the colon.

The regulatory T cell is one of many types of helper T cells. Most of these helper T cells activate an immune response and destroy foreign entities in one of two ways, either activating cells that create antibodies against foreign invaders (aka antigens) or by directly killing the cells which have been invaded by these entities. Regulatory T cells are unique in that they have the ability to block other helper T cells from activating an immune response and, thus, suppresses the immune system’s ability to combat certain foreign entities.

Although Lathrop already knew that regulatory T cells are produced and mature into immune cell activators in the thymus, he questioned if there were immature regulatory T cells in the gut, learning to tolerate commensal (normally occurring) bacteria.

All of the various T cell types have a T cell receptor (TCR), which is used to find lymphocytes that have come into contact with foreign entities. Depending on the type of T cell, it either activates the infection fighting capabilities of other immune cells or suppresses their ability to be activated. TCRs are proteins made up of 2 subunit chains, an alpha and a beta chain. The authors analyzed the alpha chains of mice that were genetically engineered to have a fixed beta chain in order to compare the alpha chains of colonic regulatory T cells and other regulatory T cells found in the colon. They found that the alpha chains of the colonic regulatory T cells were drastically different than those of the other regulatory T cells. Then, they used the same technique to compare the TCR alpha chains of regulatory T cells in secondary lymphoid organs once again to the TCR alpha chains of the colonic regulatory T cells. Just as before, they found that the TCRs of the colonic regulatory T cells differed from those found in the mice's other peripheries. These results suggest that the TCRs “learn” their responses after contact with the antigens present where they are located in the body and that the regulatory T cells are responding to antigens formed by the bacteria residing in the colon.

To determine whether the colonic bacteria were inducing the production of this colonic population of regulatory T cells, the authors created a cell line containing various types of TCRs found in the mice, including colonic regulatory T cells. These cells had the ability to fluoresce green when bonded with an antigen. The reactivity of these T cells was determined in mice with normal amounts of colonic bacteria and mice without any colonic bacteria, known as germ-free mice. The cells were only observed to fluoresce in the normal mice and the only cells that fluoresced were the colonic regulatory T cells, demonstrating that the colonic bacteria are necessary for activation of the colonic regulatory T cells.

They then attempted to identify what bacteria the TCRs of the colonic regulatory T cells were recognizing by screening pools of 2 or 3 different types of bacteria that were heat-killed after being isolated from the colonic contents of the mice. It was found that more than half of the tested colonic regulatory T cells recognized one or more of the bacterial isolates from the colon. This confirmed that the colonic regulatory T cells were responding to the bacteria in the colon.

The authors then wanted to know if these regulatory T cells were being produced in the thymus, where most T cells are known to mature, or if they developed in the colon as a response to colonic bacteria. Immature colonic regulatory T cells did not mature when placed in the thymus of the mice or in the colons of the germ-free mice, but did mature when housed in the colons of the normal mice.

The authors finally wanted to test how problems with T cell regulation and its effect on colonic bacteria may be instrumental in leading to gastrointestinal diseases. They analyzed the colonic TCR repertoires of mice expressing a fixed TCR beta chain and experiencing spontaneous colitis, a medical condition characterized by inflammation of the large intestine. Results showed that the TCRs that were normally found on colonic regulatory cells in healthy mice, were instead expressed on effector T cells which are responsible for killing foreign entities. Therefore, mice with immune cells that attacked colonic commensal bacteria developed colonic disease, showing the importance of regulatory T cells to prevent immune cells from attacking colonic bacteria.

All in all, it was found that the gut has regulatory T cells that mature in the gut and stop immune cells from initiating an immune response against the bacteria found there. It was shown that diseases in the gut, such as colitis, are caused when these bacteria are targeted for killing instead of regulation. Some questions that remain unanswered by these authors are whether the mice with colitis lack regulatory T cells in the colon causing the production of effector T cells that target the gut bacteria or if they have TCRs that do not recognize the bacteria in the colon.

Reference:

1. Guarner Fand Malagelada JR, 2003. Gut flora in health and disease. Lancet 361, 512–9

Primary Article:

Lathrop, S.K., Bloom, S.M., Rao, S.M., Nutsch, K., Lio, C., Santacruz, N., Peterson, D.A., Stappenbeck, T.S., Hsieh, C., 2011. Peripheral education of the immune system by colonic commensal microbiota. Nature 478, 250-254