Even as an infant is learning to distinguish herself from the rest of the world, a key part of her immune system is working toward the same goal, albeit in a very different manner.
From before birth through the early years of life, young immune system cells in the thymus gland are receiving an education in their native surroundings. Many of these cells, called thymocytes, will eventually mature into T cells, which will protect the body from foreign invaders like bacteria and viruses and from rogue actors like cancer cells. For now, though, they’re going through a ruthless sorting process that will determine whether they live or die.
As they pass through the massed compartments of the thymus, they’re exposed to compounds known as peptide antigens, which present the body’s signature in molecular form. If a thymocyte strongly reacts with one or more of these peptide antigens — indicating it would likely attack normal, healthy tissue if it became a full-fledged T cell — it is eliminated from the body. The thymocytes that remain form the repertoire of T cells that protect an individual over the course of a lifetime.
The peptide antigens that thymocytes encounter during their sojourn in the thymus are displayed on structural cells called stromal cells. More specifically, they’re mounted in a structure known as the major histocompatibility complex (MHC), the human version of which is the human leukocyte antigen (HLA).
A matter of timing
It was long thought that the culling of thymocytes began only once the cells developed a T cell receptor (TCR) — a structure on the surface of mature T cells that probes the MHC on other cells, looking for peptide antigens alien to the body. Cells found to carry foreign insignias are marked for destruction. Over the past half dozen years, Dana-Farber scientists led by Ellis Reinherz, MD, have published a series of studies suggesting that the selection of thymocytes for survival occurs considerably earlier, when thymocytes carry a kind of T cell receptor-in-training known as a pre-T cell receptor, or pre-TCR.
These studies, which involved a structural analysis of thymocytes, were met with skepticism by some, who held to the conventional view that the pre-TCR may play some obscure role in quality control but doesn’t interact with anything outside the cell and therefore can’t influence whether a thymocyte is retained or eliminated from the repertoire. A new study in the journal Nature led by Dana-Farber scientists tilts the argument persuasively in favor of revising the traditional view.
“Our findings suggest that the selection of an individual’s repertoire of T cells happens much earlier than was previously thought,” says the lead author of the new study, Jonathan Duke-Cohan, PhD, of Dana-Farber, who was joined in the research by Reinherz, Aoi Akitsu, PhD, Robert Mallis PhD, Patrick Lizotte, PhD, and Cameron Messier, of Dana-Farber; Jon Aster, MD, of Brigham and Women’s Hospital; Wonmuk Hwang, PhD, of Texas A&M University; and Matthew Lang, PhD, of Vanderbilt University. “The discovery not only helps answer a basic question about immune system development but may also shed light on the origins of some cancers of the thymus.”
Beta version
Each mature T cell receptor comprises two protein chains, dubbed α and β, that are unique to that T cell. The pre-TCR, by contrast, has just one unique β chain and a shortened α chain shared by all pre-TCRs. For more than 25 years, “the consensus has been that the pre-TCR could be thought of as a prototype or beta version of a TCR, enabling the cell to test out some functions before the TCR is fully assembled but not influencing the selection of the eventual TCR repertoire,” Duke-Cohan relates.
In recent studies, the research team provided structural evidence that the pre-TCR can in fact interact with the peptide antigen/MHC assembly (known as the pMHC) to adjust the size of the T cell repertoire. They also inferred that the pre-TCR’s signaling mechanism was identical to that of the mature TCR.
To answer critics who found these structural studies so contrary to the prevailing orthodoxy in immunology that they were considered unpersuasive, Duke-Cohan and his associates set out to find functional proof of an interaction between the pre-TCR and MHC-bound peptides.
“If we could show the result or effect of the pre-TCR interacting, or not interacting, with peptide MHC, it would indicate that such interactions do take place and have definite repercussions,” Duke-Cohan says.
Deceiving appearances
In the new paper, the researchers show that the absence of interaction between normal pre-TCR and pMHC in the thymus has significant and potentially disastrous consequences for the health of a mammal. “We found that if the interaction doesn’t occur, the signaling system within thymocytes goes askew,” Duke-Cohan relates. “The cells continue to change on the surface, as though they’re developing normally, but inside they remain immature. It’s as though someone was to all appearances an adult but remained a child.”
The mismatch between its exterior and interior isn’t lost on the thymocyte. “The cell can sense that something is awry — that certain genes haven’t been turned off as they should have,” Duke-Cohan explains. “It responds by reverting to an earlier stage of development — to become more like a stem cell that has the potential to develop into other types of white blood cells including granulocytes and monocytes. So it may be displaying signature characteristics of myeloid and lymphoid cell types, but inside it has the genetic wiring of a stem cell, complete with the ability to keep on dividing.”
That penchant for proliferation suggests “there’s a real risk that a tumor could develop,” he remarks. In animal models where the interaction between the pre-TCR and pMHC in the thymus was blocked, researchers found that thymic leukemias and lymphomas could develop.
“Without this early interaction, thymocytes may become vulnerable to taking the wrong pathway of development and becoming capable of forming tumors,” Duke-Cohan states. “The interaction appears to be an important checkpoint against tumor formation that wasn’t apparent until now.”
He adds that it isn’t yet known whether thymus tumors in humans form by this mechanism, but it’s possible that a subset do. “Our biological findings strongly support the structural studies showing a role for the pre-TCR in determining which T cells are selected to fight future infections and malignancies.”
The findings have particular relevance for research in cancer immunotherapies. One strand of such research involves laboratory-engineering T cells to target peptide antigens presented on cancer cells. Recent results of this work suggest that the early stages of thymocyte development need to be considered when sculpting the T cell repertoire for therapeutic purposes. Such efforts underscore the importance of basic scientific exploration in the development of new treatments for human diseases, the study authors say.