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CTLA-4Ig is Finally Making It: A Personal Perspective

2005; Elsevier BV; Volume: 5; Issue: 3 Linguagem: Inglês

10.1111/j.1600-6143.2005.00786.x

1600-6143

Jeffrey A. Bluestone,

Diabetes and associated disorders

‘What a long … strange trip it's been.’Jerry Garcia, Grateful Dead In this issue of American Journal of Transplantation (AJT), Larsen et al. have published their studies of LEA29Y/BELATACEPT, a high affinity mutant form of CTLA-4Ig that has been demonstrated to be a potent immunosuppressive drug. This is an important step forward in the development of CTLA-4Ig, a drug that has had steps backwards and sideways along the way. AJT asked me to recount my personal recollections of how we got to where we are. It was a long time ago, the spring of 1991 to be precise, when I was sitting among a few hundred immunologists at a Gordon Conference and first heard Peter Linsley present his work on CTLA-4Ig. I, like many of my colleagues, had been searching for clues to the molecular basis of T-cell tolerance, first described in 1953 by Medawar and proven over and over again throughout the next four decades. What Linsley showed at that meeting was the ability of CTLA-4Ig to bind B7 (at the time the only known ligand for CD28) and inhibit CD28 signaling. The work was a culmination of 5 years of investigation that grew out of seminal observations by Carl June, Craig Thompson and Jeff Ledbetter, who conceived of and demonstrated the critical co-stimulatory role of CD28 in T-cell activation (1). This work had stemmed from their observations that many recipients of bone marrow transplantation developed severe graft versus host disease in spite of using high doses of cyclosporine A, a revolutionary new immunosuppressant at the time that had been developed for organ transplantation at Sandoz. As part of a team at Oncogen, a Bristol Myers Squibb subsidiary, Linsley worked with Ledbetter and Ed Clark to identify the natural ligand for CD28. They succeeded, but to their dismay, a soluble form of the CD28 molecule, CD28Ig, did not effectively inhibit the interaction. Linsley was stuck. Re-enter Craig Thompson and his wife, Tullia Lindsten who had recently returned from a meeting in Israel. At that meeting, Tullia had heard Pierre Goldstein describe a molecule, cytolytic T lymphocyte antigen-4, which had no known function, but which had an amino acid sequence quite homologous to CD28. Thompson told Linsley, whom he knew via his close relationship with June and Ledbetter, about the new molecule. Linsley made CTLA-4Ig and found that it not only bound to B7 (CD80), but that it did so with a much higher avidity than CD28. As a result, CTLA-4Ig turned out to be a powerful CD28 antagonist in vitro. Sitting in Vermont, I recalled many conversations with Marc Jenkins and other young immunology colleagues at the NIH during the mid 1980s, a golden age of lunch room conversations, interactive journal clubs and great collegiality. I remembered the landmark paper of 1987 by Marc and Ron Schwartz in which they showed that engagement of the T-cell receptor by MHC-peptide, in the absence of some other signal, led to T-cell anergy—the equivalent of tolerance in a tissue culture plate (2). As I sat there, it hit me—if CD28 was part of the other signal, maybe we could use CTLA-4Ig to generate tolerance in vivo. On the plane back to the University of Chicago I wrote a long letter to Linsley, proposing my idea and pleading for some of his precious reagent. I was stunned when he responded within a week, and soon generously sent me 5 mg of the protein. I held a project meeting with Debbie Lenschow, an M.D., Ph.D. student, to plan our experiments. Debbie, working with an excellent microsurgeon in the lab, Yijun Zeng and Dick Thistlethwaite, demonstrated that CTLA-4Ig blocked islet xenograft rejection and also led to long term antigen-specific tolerance (3). We worked with human islets since the CTLA-4Ig we got from Linsley was human. We were not the only ones working on this. Larry Turka, along with Craig Thompson, had pursued similar studies in an allogeneic system and published some beautiful studies demonstrating similar effects of the drug (4, 5). I remember telling Craig that we were done … we finally had our perfect immunotherapy … a drug that did not affect every T cell (like Cyclosporine A or anti-CD3), but only those that had received a “signal one”; only those encountering transplant antigens … and it induced tolerance no less. Science, of course, is never so simple. The excitement of the time could not have foreshadowed the 10-year rollercoaster ride CTLA-4 had in store for us. Among the more significant setbacks was the discovery of a second ligand for CD28/CTLA-4–CD86 (B7–2) (6). Unfortunately, CTLA-4Ig bound CD80 much better than CD86. In addition, as Sayegh and Turka showed, the drug actually did not induce anergy in vivo so we really did not know how it worked (7). Moreover, short-term treatment of non-human primates with CTLA-4Ig showed little efficacy (8). The disappointing results with CTLA-4Ig, coupled with new excitement over CD40/CD40L interactions, moved the field towards this hot new co-stimulatory pathway (8). Meanwhile, on top of all this, the legal battles seemed endless, as the University of Michigan, Repligen and BMS fought each other over ownership. The only bright spot was that early psoriasis trials showed strong clinical efficacy, but only at doses (40 mg/kg) that would turn out to be a production challenge for BMS (9). By 1999, I had come to believe that the drug was dead. Fortunately, the BMS group, led by Robert Peach, stepped back in, making a higher affinity CTLA-4Ig–LEA29Y (now designated belatacept). This molecule binds CD80 twofold better than CTLA-4Ig and, more importantly, binds CD86 fourfold better. In large animal studies, the avidity of the new agent produced greater efficacy, setting the stage for the corporate buy-in from BMS to develop LEA29Y/belatacept as a drug. Simultaneously, studies using long term co-stimulation blockade demonstrated increased efficacy in the large animal studies (10). On the other hand, unexpected side effects of anti-CD40L antibodies had become evident, diminishing enthusiasm for this approach. Which, today, brings us almost full circle, back to the excitement over CD28/B7 as a viable therapeutic pathway. As highlighted in the paper by Larsen et al., in this issue, the rational development of LEA29Y/belatacept has validated the original notions of co-stimulatory blockade and suggested that a more specific, less toxic immunotherapy is on the horizon. Of course, we really do not know if LEA29Y/belatacept will be better clinically than CTLA-4Ig (which continues to be developed as a drug for autoimmunity). Both drugs are currently in phase II and III trials in autoimmunity and transplantation. Currently, the development of CTLA-4Ig and LEA29Y/belatacept is aimed at achieving immunosuppression. In my opinion that should be only the first step. The development of immune tolerance remains an important and attainable goal. Drugs like LEA29Y/belatacept have the potential to move us toward this goal. Moreover, combination therapies, perhaps targeting CD40, ICOS, LFA-1 or other cell adhesion/co-stimulatory pathways, may induce a more robust tolerogenic effect. Drug withdrawal studies are a critical next step and, as a community, we must continue to press for continued development of such drugs, for sometimes neglected indications, and perhaps in combination therapies that may yield greater insight into the nature of immune tolerance. Do not stop now. The story of CTLA-4Ig has had many contributors. If it were not for the perseverance of many scientists in industry and academia (many of whom could not be included here due to space constraints), LEA29Y/belatacept would not be a drug in development today. I would like to acknowledge all of these contributors, especially my own colleagues and students—Deborah Lenschow, Yijun Zeng, J. Richard Thistlethwaite and Phil Padrid—whose dedication helped move this field forward. Finally, I would like to thanks Hugh Auchincloss for his help and advice in the writing of this piece.