Mystery Rays from Outer Space

I’ve previously posted on regulatory T cells (TRegs) and their potential role in transplants. Briefly, TRegs are capable of specifically shutting off immune responses to particular antigens; they’re normal components of an immune system. TRegs can be damaging in some contexts — for example, in cancer, where it seems that TRegs often shut off immune responses to tumors, so that the tumor can escape immune clearance; and they can be beneficial in other context — for example, in some persistent virus infections, where a chronic immune response would be damaging, TRegs apparently modulate the immune response so that the virus persists but doesn’t cause severe damage.

There are a couple of obvious scenarios where it would be nice to be able to control TRegs. There’s a lot of interest in reducing TReg activity in cancer, such as with CTLA4 antagonists. There’s also a lot of interest in increasing TReg activity in organ transplants, and there have actually been a couple of cases where it’s seemed to have worked.

A recent paper in PNAS¹ offers steps toward a more general procedure, that could in theory lead to controlled, planned generation of TRegs for any antigen.

A key aspect of TRegs is that they are antigen-specific. They don’t randomly suppress immune responses; they identify particular antigens that should be tolerated, and shut off immunity to those antigens. That allows fine control over the response, but it also makes it harder to catch a TReg; T cells (not just TRegs) that recognize any particular antigen are very rare events, hiding in a blizzard of other specificities. What if you could force T cells for an antigen you choose to enter the TReg pathway?

This has already been done, in fact, but in a very artificial system — in mice with transgenic T cell receptors. These mice overwhelmingly express a single TcR in all of their T cells — there’s no snowflake in a blizzard problem, because the entire blizzard is made of identical flakes. Harold von Boehmer’s group has shown that you can drive these transgenic T cells into the TReg pathway by offering very, very low levels of antigen, under defined conditions, over a long period. ² The recent paper¹ shows that you can do the same thing in normal, non-transgenic, mice; and by doing this you can force graft tolerance. (They used female mice and drove tolerance to the male antigen H-Y antigen. The tolerized female mice then became tolerant of male grafts, while the control female mice rejected the male grafts.)

The key, at least for this particular protocol, seems to be to use very low dose antigen and “suboptimal” conditions (where “optimal” refers to conditions that drive conventional immune responses. The vocabulary of immune responses is really kind of misleading, because it’s focused on easily-measured responses like protection against viruses or graft rejection. Regulatory T cell responses are just as active, and probably are just about as common and important, but it’s hard to talk about them without giving the impression that they’re somehow passive, or abnormal, or defective).

One problem with moving this into the clinic is that you would need to know what the target antigen is, which in an outbred population like humans you do not know a priori. However, as bioinformatic and experimental techniques for identifying antigen peptides improve, it may become more practical to run this for patients before their transplants. The potential payoff would be very high, because you might be able to remove immunosuppression altogether:

If a procedure as simple as peptide infusion, which permits de novo induction of Tregs from mature T cells, prevents transplant rejection or GVHD, it could offer a realistic opportunity to induce tolerance to a variety of antigens such as allergens, transplantation antigens, and antigens causing autoimmunity while minimizing undesirable side effects often associated with general immunosuppression.

1. Verginis, P., McLaughlin, K.A., Wucherpfennig, K.W., von Boehmer, H., Apostolou, I. (2008). Induction of antigen-specific regulatory T cells in wild-type mice: Visualization and targets of suppression. Proceedings of the National Academy of Sciences, 105(9), 3479-3484. DOI: 10.1073/pnas.0800149105[↩][↩]
2. Kretschmer, K., Apostolou, I., Hawiger, D., Khazaie, K., Nussenzweig, M. C., and von Boehmer, H. (2005). Inducing and expanding regulatory T cell populations by foreign antigen. Nat Immunol 6, 1219-1227.
   Apostolou, I., and von Boehmer, H. (2004). In vivo instruction of suppressor commitment in naive T cells. J Exp Med 199, 1401-1408.[↩]

Last week I talked about regulatory T cells (TRegs) in cancer. TRegs are often abundant in tumors, and have been linked to poor outcome, presumably because they prevent immune rejection of the tumor. The obvious flip side of this would be in a situation where you want to prevent immune rejection — in organ transplants. TRegs have been frustratingly hard to harness for this, though. (”Tolerance is the future of transplantation, and always will be.” –Norman Shumway)

A paper in the New England Journal of Medicine¹ describes an encouraging step forward in this, achieving something that is at least close to the holy grail of transplantation — organ transplants that are maintained indefinitely without immunosuppression. Normally, organ transplants are rejected by the immune system, unless they’re from an identical twin (in which case the donor organ is perceived to be “self” by the immune system). By suppressing immunity, organ transplants can “take” without rejection; usually the immunosuppression is fairly harsh at first, but can be eased up over time, suggesting that a certain degree of tolerance is reached. (Also, the donor organ probably becomes less immunogenic over time, as some of the most immunogenic cells move out of the graft or die off, leaving less immunogenic tissues behind.)

Even though today’s immunosuppression is relatively gentle and focused, it’s only gentle relative to previous brutal treatments; it still leaves the recipient susceptible to infection, so there’s always a juggling act, balancing risk of rejection with risk of infection. The goal, then, has long been to find techniques that will allow the recipient’s immune system to become tolerant of the donor organ, as is seen in tumors.

The paper describes five kidney transplants that were preceded by bone marrow transfer from the donor. In four of the five cases, they were able to withdraw immunosuppression altogether, and the transplant wasn’t rejected (for at least one to five years, and counting). This is particularly exciting because these transplants weren’t from HLA-matched donors, meaning they were fairly immunogenic. (The same group, and another paper in the same issue of New England Journal, have done the same thing with HLA-matched transplants,² which is still pretty interesting; but partially-mismatched transplants are much more common these days. )

One particularly interesting observation is that the bone marrow transfer only led to temporary chimerism (i.e. the donor bone marrow didn’t take permanently, and after a while only the original recipient bone marrow cells were present); but the tolerance persisted. They were able to find lots of TRegs infiltrating the donor kidneys, though, and so they believe that the long-term tolerance is probably because of TRegs (peripheral tolerance) although in the early stages thymic effects (central tolerance) may have been more important.

The same issue of New England Journal describes the case of a young liver transplant recipient who apparently had her bone marrow seeded with stem cells from the donor liver, resulting in a switch of blood type and immune system to the donor’s and, again, a complete take of the graft without immunosuppression.³ That’s the case that’s getting all kinds of press right now, but while it may turn out to be an important guide to future treatment, it was essentially pure luck — the other cases here were the result of deliberate planning and defined conditions, which means that they can be repeated; the flashy case can’t, yet.

1. Kawai, T. et al., 2008. HLA-Mismatched Renal Transplantation without Maintenance Immunosuppression. N Engl J Med, 358(4), p.353-361. [↩]
2. Bühler, L.H. et al., 2002. Induction of kidney allograft tolerance after transient lymphohematopoietic chimerism in patients with multiple myeloma and end-stage renal disease. Transplantation, 74(10), p.1405-9.
   Fudaba, Y. et al., 2006. Myeloma responses and tolerance following combined kidney and nonmyeloablative marrow transplantation: in vivo and in vitro analyses. American journal of transplantation, 6(9), p.2121-33.
   Spitzer, T.R. et al., 1999. Combined histocompatibility leukocyte antigen-matched donor bone marrow and renal transplantation for multiple myeloma with end stage renal disease: the induction of allograft tolerance through mixed lymphohematopoietic chimerism. Transplantation, 68(4), p.480-4.
   Scandling, J.D. et al., 2008.
   Tolerance and Chimerism after Renal and Hematopoietic-Cell Transplantation. N Engl J Med, 358(4), p.362-368. [↩]
3. Alexander, S.I. et al., 2008. Chimerism and Tolerance in a Recipient of a Deceased-Donor Liver Transplant. N Engl J Med, 358(4), p.369-374. [↩]

Stuffed with duck as I am (we don’t do turkey for Thanksgiving in our house) I’m not up to a long post, but I thought a paper in the latest issue of Science was pretty cool. The paper is
Czechowicz, A., Kraft, D., Weissman, I. L., and Bhattacharya, D. (2007). Efficient Transplantation via Antibody-Based Clearance of Hematopoietic Stem Cell Niches. Science 318, 1296-1299. ¹

Very briefly, they show that one of the obstacles to bone marrow grafts — even in the absence of host-versus-graft immunity — is that there are a limited number of niches for hematopoietic (bone marrow) stem cells. The native stem cells occupy those niches, so injecting in a donor’s stem cells is very inefficient; only a tiny number can find a home and supply new, desirable progeny. If I’m interpreting the data right, there seem to be only a few hundred open slots, out of maybe 25000 total slots, available for donor stem cells.

They came up with a protocol that transiently and specifically eliminated host stem cells — opening up niches — before the graft, and the results were pretty dramatic. Without treatment, the chimerism rates (indicating efficiency of donor engraftment) was around 3%; with treatment, it was 90%. If this works in humans as it does in mice, it offers a much gentler alternative to the really brutal and toxic treatments that are usually necessary today.

So what does “niche” mean, in this context? Is it a physical slot into which the tab of a hematopoietic stem cell is tucked? Is it a conceptual niche, a constraint based on available levels of some soluble factor, or on rates of contact with some supporting cell type? I think all three are possible, and² have parallels in other aspects of the immune system.

For example, growth in the thymus (the figure at top left) probably requires physical niches, cells into which developing thymocytes cuddle up and receive nourishment and advice as they mature. (See, especially, the videos taken by Bousso et al, ³ of thymocytes interacting with thymic stromal cells.) Although I admit I find that the most attractive concept, I don’t really have a good reason for it, and there are probably good examples of non-physical “niches” as well. Survival of naïve lymphocytes outside of the periphery requires intermittent contact with MHC class I molecules — potentially a limiting factor, if they have to compete with others of their kind. There are also several examples of regulation by limiting amounts of certain cytokines, such as IL-7. ⁴ Still, the various two-photon microscopy videos of in-situ interactions that have been coming out over the past few years have really made me appreciate the importance of physical location and direct interactions in the immune system, which might explain my bias toward physical niches.

1. As an experiment in aggregation, I am including a second version of the reference here, thus: Czechowicz, A., Kraft, D., Weissman, I.L., Bhattacharya, D. (2007). Efficient Transplantation via Antibody-Based Clearance of Hematopoietic Stem Cell Niches. Science, 318, 1296-1299. DOI: 10.1126/science.1149726[↩]
2. According to current understanding, anyway[↩]
3. Bousso, P., Bhakta, N. R., Lewis, R. S., and Robey, E. (2002). Dynamics of thymocyte-stromal cell interactions visualized by two-photon microscopy. Science 296, 1876-1880. [↩]
4. Purton, J. F., Tan, J. T., Rubinstein, M. P., Kim, D. M., Sprent, J., and Surh, C. D. (2007). Antiviral CD4+ memory T cells are IL-15 dependent. J Exp Med 204, 951-961. [↩]