I don't think I'm alone in my prurient interest in research related to sex, since those posts tend to be wildly popular. At the same time, I try mostly to stick to my field of expertise while blogging, and the intersection between sex and the immune system doesn't happen that often (unless we're talking about sexually transmitted infections). Even then, I'm writing for a food blog now, and the overlap in the sex/immunology/food venn diagram is even smaller. So when I read that mice eating yogurt had altered immune systems which led to increased testicle size, I almost couldn't believe my luck!

Probiotic Microbes Sustain Youthful Serum Testosterone Levels and Testicular Size in Aging Mice (Open Access)

Actually, it's not yogurt, but a particular bacterium often found in yogurt that causes the effects - mice fed Lactobacillus reuteri in their drinking water had larger testes than mice not given any probiotic. The results weren't that huge (heh) - only 5% - but they used a lot of mice, and there's not that much natural variation, so the result seems real. They also performed the important control of feeding mice a different bacterium (E. coli), and saw no difference.

That's the testicle headline grabbing part of the story, but it's only the beginning, and I actually found the rest of the paper more compelling. Normally, as mice (and people) age, testosterone production drops, largely because of the degeneration of a particular cell type called a "Leydig Cell," that's found in the testes. Decreased testosterone has been implicated in a number of the problems of aging in men (though based on a cursory review of the literature, it's not clear to me if those studies are definitive), and some doctors have suggested hormone-replacement therapy in men that have particularly low levels.

Interestingly, these researchers found that mice given the probiotic bacterium maintained their testosterone production at pretty substantial levels into old age (which is 5 months for a lab mouse), and this was correlated with increased numbers of healthy Leydig cells, where their untreated partners's Leydig cells atrophied. But how could a bacterium fed to mice in their drinking water decrease aging of the testes? Here's where the immune system comes in (and for the record, this is yet another example of my mantra, the immune system is involved in every aspect of health).

This lab is really an obesity lab, and their previous studies have shown that certain probiotics can reduce the health problems associated with obesity by decreasing the production of a cytokine - a chemical messenger of the immune system - called IL-17. Normally, IL-17 is produced during an inflammatory response to deal with bacterial infections, but over production can cause tissue damage and is associated with a bunch of autoimmune disorders. The link between obesity and inflammation, has been well established, and this lab's work has shown that probiotics can help with that.

They wondered if IL-17 might play a role in their gonad findings, and sure enough, blocking IL-17 with an antibody mimicked all of the effects seen with the probiotic. Which is cool, but ultimately unsatisfying. They don't show that inflammation is involved in the atrophy of Leydig cells or do any experiments to establish the mechanism. I want to know why older mice are making IL-17 that's killing their testes, and how these bacteria are stopping it.