It is easy to see how mice, monkeys, or other mammals might have something to tell us about human biology. But sea squirts? Animals so low on the evolutionary tree that they don't even have backbones? What can they reveal about complex human medical problems?

As it turns out, humans have an evolutionary connection with these creatures. Our most distant chordate (backboned) ancestors were similar to primitive sea squirts. Certain characteristics of sea squirts might, therefore, represent early forms of similar characteristics in humans.

More recent work has identified sea squirt homologues of genes whose mammalian counterparts participate in immunity (for examples and reviews, see Refs. 1-6)

Let me give you an example. When two colonies of Botryllus touch, their blood cells mix and a kind of natural transplantation occurs. This is followed by one of two consequences. Either the colonies fuse, producing a single organism, or the cells recognize each other as being too different and the colonies grow apart after a rejection reaction.

Humans also have the ability to accept or reject transplanted tissue. This is a function of the immune system, an elaborate network of blood proteins and specialized cells that protect the body against infectious disease and tumors. In humans, cells from every individual carry a set of unique surface markers, or transplantation antigens, that define those cells as belonging to that individual alone. By reading these markers, the immune system distinguishes self from non-self, and in this way decides to accept or reject a transplant. Sea squirts distinguish self from non-self in much the same way.

Intercolony fusion and rejection in sea squirts is mediated by blood cells, which recognize and respond to foreign histocompatibility determinants using a "missing self" mechanism similar to that employed by mammalian natural killer or NK cells that recognize MHC class I molecules (Ref. 7).

Because I was curious whether transplantation in sea squirts might represent an early form of human immunity, I traveled to the MBL in 1980 to conduct research on Botryllus. It wasn't long before my colleagues and I confirmed that transplantation in these animals is controlled, as it is in human grafting, by defined genes that can be studied in a laboratory.

Given our evolutionary link to sea squirts, these genes might actually be simple versions of those that control human grafting. Studying how they work might tell us something about their human counterparts, and might there, help solve puzzling problems of human biomedicine - problems like AIDS

AIDS is a disease of the human immune system, but the story of how sea squirts won a role in AIDS research begins not with immunity but with fertilization.

Like us, Botryllus, and other sea squirts begin life as a single fertilized egg. After fertilization, this egg cleaves, maker more and more cells, and ultimately develops into a larva that looks somewhat like a frog tadpole. These animals have a peculiar property that sets them apart from us or any other true vertebrate: each individual animal is hermaphroditic, producing both sperm and eggs.

Theoretically, these animals should be able to self-fertilize' but this rarely happens. Sea squirts employ the same principles to prevent self-fertilization between close relatives as they employ in their control of transplantation. Thus, just as the blood cells of two related Botryllus colonies will not bind to and kill each other, the sperm from an animal cannot bind to and fertilize it's own eggs.

This suggested to us a startling possibility; not only might the principles of transplantation and fertilization in these creatures be similar, but the transplantation markers involved in the process might be similar as well.

To test that possibly, the direct experiment seemed best: mix sea squirt sperm cells with blood cells. Would the sperm bind to the blood cells? Would they distinguish between blood cells from the same animal (self) and a different animal(non-self)?

The answer is yes. Just as in the case with sperm and eggs, if you take blood and sperm from animal A and mix them together, the sperm will not bind to self blood cells. The same is true for animal B. But if you mix sperm from animal A with blood cells from animal B - or sperm from B with blood from animal A - the sperm will bind to and penetrate the other animal's blood cells. Using the electron microscope, we can see their heads inside the cells. We were very surprised at how clear the result was.

Out of curiosity, we decided to conduct the same experiment with human sperm and blood cells. No one had ever done an experiment of the kind with this objective in mind. When we mixed blood cells from a normal donor with sperm from a normal donor, the sperm bound to and penetrated the blood cells.

As in sea squirts, it turn out the receptor for sperm - the substance on the blood cells to which the sperm sticks - is a self-recognition molecule. In the human studies, we know a little more about it's identity; it is a structure called HLA-DR. HLA-DR is a blood cell specific transplantation molecule found on many white blood cells. It is important in many diseases, including AIDS.

AIDS is a disease in which the immune system simply goes down and disappears. It is caused by an identified virus, HIV, which can be passed through semen in sex. No one knew whether the virus was contained in the seminal fluid, or in the leukocytes (a kind of white blood cells) present there, or if the sperm played a role. There had been a general tendency to discount the sperm as a candidate for the virus carrier, but our data now strongly suggests a primary role for sperm in the transmission of AIDS.

Remember that sperm binds to HLA-DR on white blood cells. Interestingly enough, it turns out that the AIDS virus is covered with a protein coat that mimics HLA-DR in several important respects, including its structure. This suggests that sperm are able to attract and acquire AIDS virus particles.

When we tested this hypothesis experimentally, we found that the AIDS virus binds to the sperm surface. The finding has been confirmed (10-14). Even today, relatively little is known about the biology of HIV transmission from semen during sexual contact. recent work has shown virus particles or intermediates to be present in testicular germ cells, seminal fluid, seminal leukocytes, and sperm of HIV-infected men. Of these potential sources of infectious HIV, however, only seminal leukocytes have been shown conclusively to be infectious for target cells in vitro (15). Although HIV-bearing sperm may transfer infection to target cells (or even to eggs (10), a primary virus vectoring role for sperm seems unlikely because few such sperm are present in infected donor ejaculates (5% or fewer of total sperm, (13) HIV-gamete interactions will be the subject of an Ares-Serono Foundation Workshop to be in Siena, Italy in October of this year (1997). Knowing that sperm can penetrate blood cells, we could see that this might be a very fast and efficient way for a virus to get into cells: carried by the sperm. When sperm from donors with AIDS are combined with white blood cells from a healthy donor, in many cases we can detect infection of those blood cells within two to three weeks of their exposure to the "infected" sperm.

Although we reported that purified sperm from infected donor semen infected target cells in vitro (16-18) we could not eliminate the possibility that the sperm preparations contained sub-detectable numbers of infected seminal leukocytes (15). We also reported that HIV was readily adsorbed and bound to normal sperm, and that such sperm transferred infection to target cells (16-18), but again, we could not exclude the possibility that undetected free virus was present in the sperm pellet (V. Scofield and M.H. Lee, unpublished). If we assumed that contaminating cells or virus were responsible, however, it was necessary to explain the unexpectedly high infectivity of the sperm-virus inocula, because virus present after washing of the sperm was below the limits of detection by standard essays.

Although we reported that purified sperm from infected donor semen infected target cells in vitro (16-18) we could not eliminate the possibility that the sperm preparations contained sub-detectable numbers of infected seminal leukocytes (15). We also reported that HIV was readily adsorbed and bound to normal sperm, and that such sperm transferred infection to target cells (16-18) but again, we could not exclude the possibility that undetected free virus was present in the sperm pellet (V. Scofield and M.H. Lee, unpublished). If we assumed that contaminating cells or virus were responsible, however, it was necessary to explain the unexpectedly high infectivity of the sperm-virus inocula, because virus present after washing of the sperm was below the limits of detection by standard essays.

When we used quantitative methods to assess infectivity in relation to virus content, we found that virus-incubated normal sperm were significantly more infectious for target cells than are equivalent amounts of virus without sperm. (19) It appeared, therefore, that virus inocula containing sperm were more infectious that those that did not, whether or not the virus was actually carried by the sperm themselves. This suggested that sperm/HLA-DR interactions affect target cells in a way that renders them more susceptible to HIV infections.

The nature of this effect may derive from cell activation events triggered by sperm engagement of HLA-DR (20) When sperm bind to lymphoid cell HLA-DR molecules, they activate HLA-DR coupled signal transduction mechanisms that affect target cells physiology (21). In this effect, sperm resemble bacterial superantigens, which also crosslink HLA-DR (22) and which promote HIV infection by activating viral target cells (23)

If sperm engagement of HLA DR increases lymphoid cells susceptibility to HIV, it may promote genital-mucosal transfer of virus from semen. If so, such events may be critical parts of the cellular microenvironment.

What does this mean in terms of preventing AIDS or slowing the epidemic? What it suggests is very simple: if the sperm are primary vehicles for transmission of the AIDS virus, treatments that effect the sperm should help stop the transmission and the epidemic.

In collaboration with other research groups, we now are conducting epidemiological studies using groups of people who are at risk for AIDS to determine whether vasectomy can prevent transmission of the disease. What we started here at the MBL ten years ago as zoology research with sea squirts has become a public health project aimed at stopping an epidemic.

These developments illustrate the critical importance of basic marine biological research in biomedicine. By offering unusual ways of looking at perplexing problems, this kind of research will continue to be a necessary part of the effort towards control of human disease.