Feature: The delicate immunological negotiation of sex

Considering that humans have an immune system finely tuned to combat foreign pathogens, the female reproductive tract is remarkable in that it can tolerate, and literally nurture, a new human life when half of that life was created from a totally foreign source: the male gamete. The question is: how?

Reproductive immunology can be viewed as a branch of mucosal immunology, which deals with epithelial linings in the body such as the respiratory and intestinal tracts. These surfaces have evolved a specialised immune response designed to protect against pathogenic invasion and yet also exhibit a level of immune tolerance enabling them to encounter all that the outside world throws at them without triggering the entire immune arsenal.

Originally a mucosal immunologist, Professor Sarah Robertson decided to shift focus many years ago to specialise on the female reproductive tract, and particularly to address one of the most fascinating immunological questions around: pregnancy.

According to Robertson, what reproductive tissues have much in common with other mucosal sites, such as the lung and gut, is the problem of having to deal with the usual pathogens, and discriminating them from the harmless stuff. However, the antigenic challenges to the female reproductive environment and the immune decisions that go on there are, in one very important feature, quite unique.

“It has to allow immune tolerance, firstly of the male gamete in seminal fluid and, ultimately, of the foetus and placenta during pregnancy. I thought it was enormously interesting that this tissue has to be both tolerogenic and retain the capacity to defend itself against dangerous invasion or challenge,” says Robertson.

It was this challenge, and how the female body deals with it, that drew Robertson to want to understand more about this special immunological environment. It was also clear that a better understanding of that process could potentially help women who suffer fertility problems and pregnancy-related pathologies.

Early hypotheses

“The history of reproductive immunology has been very much informed by, and interconnected with, transplantation immunology,” says Robertson. “Specifically, Medawar’s hypotheses to explain pregnancy has prevailed for 50 years or more.”

Nobel Laureate, Sir Peter Medawar, was a British biologist whose work on tissue graft rejection, and the discovery of acquired immune tolerance, was pivotal to the field of transplantation as we know it. He was awarded a Nobel Prize in 1960 together with Sir Frank Macfarlane Burnet for this work.

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“Medawar’s views on how the conceptus tissue manages to evade rejection argued for either a physical separation of the immune system from the conceptus tissue or, alternatively, ignorance to the antigenic information present in the conceptus. In other words: just not mounting a response at all,” says Robertson.

“His explanations for these hypotheses included the female reproductive tissue not expressing the antigens presented by the male gamete and foetus even though the genes were there, whereby the antigenic information would virtually be invisible to the immune system. Alternatively, the mother’s immune response at that site was in some way systemically suppressed during pregnancy.”

The trouble was that, for many years, the whole field of reproductive immunology looked for evidence to sustain Medawar’s hypotheses, but nothing emerged. “And it turns out that what happens is almost exactly the opposite,” she says.

The female immune response in the reproductive tissue is not rendered ignorant or suppressed to establish pregnancy. In fact the converse in true, says Robertson, with the immune system very intimately associated with everything that goes on in the reproductive process.

“The truth started to become apparent when two or three groups around the world were looking at T cell responses during pregnancy, and found nothing to support Medawar’s ideas,” she says. The advent of T cell transgenic approaches in the last several years cemented the idea that what Medawar (and the whole field) had thought was actually incorrect. Researchers consistently found that instead of an absence or failure of T cell response to the conceptus, the systemic T cell response in pregnancy is very strong.

The other thing that changed the field was the re-emergence of regulatory T cells (Treg) and the roles they play in facilitating immune tolerance at mucosal surfaces. These cells work by suppressing the immune responses of other cells so things don’t get out of hand, and a key publication in 2004 demonstrated that Treg cells are a physiological necessity for pregnancy to occur.

“We thought right from the beginning of looking at this question, that to understand what is going on we would need to go right back to the earliest stages of the process, when the mother is first coming into contact with the antigenic information from the embryo. We assumed that this time of embryo implantation would be critical,” says Robertson.

“However, in studying this hypothesis we discovered very potent immune activation occurring even prior to implantation. In fact, the ball really gets rolling several days earlier, around coitus and the peri-conceptual period when the oocyte is fertilised.”

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Indeed, Robertson’s team found exciting new evidence several years ago that the immune environment of the female reproductive tract during the 12 hour period following coitus has an enormous capability for turning on immune responses. There is major recruitment of antigen-presenting cells (dendritic cells and macrophages) and strong upregulation of immune-activating molecules.

“This seemed completely at odds with what we were expecting,” says Robertson. “We thought that everything was being downregulated and turned off for the embryos to come along. But instead, every parameter we measured was upregulated and ready to activate a new immune response.”

It also became apparent to Roberston’s team that the good old Treg cells play a key role at this early stage. “We formulated the hypothesis that antigenic information from the male seminal fluid is needed prior to pregnancy to activate and expand Treg populations in the reproductive environment.”

So then, by the time the embryo ‘invades’ the female uterine tissues during implantation – where it comes into very close contact with the maternal blood and all its immune arsenal ready to reject foreigners – the Treg machinery is already turned on, preaching tolerance. This was the first such evidence of a major role for seminal fluid in the process of establishing maternal tolerance, and Robertson’s findings were pretty out there at that time.

“People thought that sperm were just all about fertilising the egg and that would be the end of the male contribution to establishing pregnancy. But what we showed is that the seminal fluid has this ‘priming’ function in the reproductive process that we had not really considered previously: to organise and prepare the female immune response in readiness for implantation.”

The right investment choice

The next step was to consider why the reproductive immune response works this way. What is the evolutionary benefit? “If you just want to maximise the chance of establishing pregnancy at every coital event, Medawar should have been right, and it would be easier just to keep all of these immune cells out of it,” says Robertson.

The team’s alternative explanation is quality control. “We know that a large number of implanted embryos are genetically unstable or abnormal, and probably more than that prior to implantation, and there are a variety of other reasons why the conditions may not be right for the female to invest reproductive energy in a particular male partner at that point in time.

“It therefore makes sense that the female immune system empowers the reproductive system with the capacity to be choosy so early in the process.”

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In looking at what parameters the reproductive system may use to discriminate what will make a successful pregnancy, Robertson’s group did a series of studies involving T cell transgenics and cytokine analysis.

From these, it seems that the Treg cell populations induced at the very earliest points of the process are educating the female immune environment to preferentially activate either the ‘reject’ or ‘tolerate’ response. “Thus, the cytokine environment of the female reproductive tract at the time of coitus is the main determinant of the consequences.”

The female immune environment is, of course, continuously influenced by a multitude of factors endogenous to the female – nutrition, stress, infection – and all of those things are going to skew the cytokine environment towards a rejective response to the antigenic information in the male gamete.

Meanwhile, the male seminal fluid is also providing a bunch of factors, like TGFbeta, that try to convince the female reproductive tract that tolerance is the way to go (the cellular version of chatting-up).

“So, it will be the net result of the pro-tolerant signals coming from the male input and the potentially anti-tolerance signals existing in the female that will determine the fate of the embryo in terms of immune response.”

Understanding what influences that T cell decision during the periconceptual period, they provide new clues for improving fertility for couples with problems establishing pregnancy, according to Robertson.

“We also know that many pathologies of pregnancy, such as recurrent miscarriage and preeclampsia (when the placenta is not functioning correctly), are linked to immune events that occur right at the outset of pregnancy and in this periconceptual period.

“So getting that immune response right in the beginning influences not only whether you get a pregnancy but also the health of the foetus and that then has implications for the health of the child after birth. Indeed, it is increasingly evident that some of these pathologies can cause chronic health problems later in life so making the right (or wrong) decision first up can have consequences for the health of the population as a whole.”

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Into the future In ongoing work, Robertson’ team plans to continue investigating how the seminal fluid communicates to the female environment and immune system during pregnancy and, in particular, trying to drill down to the subtleties of what regulates it all.

MicroRNAs are one such suspect, with the idea that sperm are delivering more than just cytokine protein messengers to advocate for female tolerance.

”There must be other more sophisticated communication pathways that cells use, and that we are only just beginning to learn about. We knew already that microRNAs are present in sperm, although not what it was doing there.

In preliminary work we can now find microRNA delivered to the epithelial cells in the female tract after coitus and actively changing gene expression in those cells.

So, that is potentially another kind of persuasive signalling coming from the male that is acting sort of like a virus, trying to hijack the female reproductive environment and convince the immune system towards conception with that particular male.” Depending on the strength of the signal, the female may or may not comply.

This sort of process, called ‘female cryptic choice’, is already known about in flies and crickets, where it acts to figure out if a given male is of sufficiently high quality to warrant the female’s reproductive investment.

Robertson’s results indicate that similar things are occurring in mammals and that such a process might involve a whole host of mechanisms including the Treg cells in the female and signals from the males such as TGFbeta and microRNAs.

“It looks like the female reproductive system has a competence that we didn’t give it credit for in being able to be choosy about potential fathers and have in-built discrimination mechanisms from so early in the pregnancy process. I call it immune-mediated quality control.” Of course, just over half of you are probably not surprised by this notion at all.