CD25-TARGETED THERAPY REVISITED (2000)

Functional Responses of T Cells Blocked by Anti-CD25 Antibody Therapy During Cardiac Rejection. Transplantation 2000; 69: 331. Baan CC, van Gelder T, Balk AHMM, Knoop CJ, Holweg CTJ, Maat LPWM, Weimar W. Back in 1986, we concluded in one of our research articles: “These results support the important role of the interleukin 2 receptor (IL-2R)-bearing cells in the mechanism of allograft rejection; they may represent an important target for immunosuppression in clinical organ transplantation” (1). Indeed, T-cell responses regulated by the IL-2/IL-2R pathway have often been portrayed as central in the allograft rejection cascade. By targeting only those T cells that are committed to participate in undesired immune reactions, IL-2R–targeted therapy may well represent a more specific therapeutic approach that should not affect normal immune surveillance in transplant recipients. However, the initial excitement and promising results from animal studies were soon followed by often conflicting clinical data (2). It is only the last few years, when our better understanding of the IL-2R structure/genomic architecture and of the interlocking gears between the IL-2R complex and host cytokine networks, that have provided impetus for and much needed insights into IL-2R–targeted therapy. Indeed, a number of recent clinical trials have clearly shown that induction treatment with genetically engineered anti–IL-2R antibodies may significantly reduce the frequency of early acute rejections (3), confirming the importance of the IL-2/IL-2R pathway in transplant recipients. The IL-2R represents a multimeric complex of at least three transmembrane proteins of different IL-2–binding affinities: a 55-kDa α chain (also referred to as CD25 or Tac), a 75-kDa β chain, and a 64-kDa γ chain (Fig. 1A). The functional high-affinity IL-2R complex is composed of noncovalently bound α, β, and γ chains. This receptor is present on almost all activated but not resting T cells, and the interaction between IL-2 and its high-affinity receptor is required for intracellular signal transduction, clonal expansion, and continued viability of activated T cells. It is becoming increasingly appreciated that intercellular communications can be mediated by cytokines that show a high degree of redundancy. The redundancy may be explained by the sharing of common receptor subunits among the members of the cytokine receptor superfamily. A number of T-cell growth factors (TCGFs) including IL-2, IL-4, IL-7, IL-9, and IL-15 utilize the same IL-2R γ chain, as an essential signal-transducing component (Fig. 1B). Interestingly, a vigorous classical T cell-dependent rejection of islet allografts can still be mounted in mouse recipients devoid of both IL-2 and IL-4 (4). Indeed, non–T cell-derived TCGFs, especially IL-7 and IL-15, can be actively involved in supporting the IL-2/IL-4–independent allograft rejection response, suggesting that efficient immunosuppressive therapy should target the common receptor components (e.g., IL-2R γ chain) of TCGFs. Figure 1: (A) The human IL-2R complexes and their affinities for IL-2 binding. (B) Sharing of the receptor subunits between IL-2, IL-15, and IL-7.The article by Baan et al. in this issue provides important insights into putative mechanisms of graft rejection despite concomitant blockage of the IL-2/IL-2R pathway. The authors hypothesize that IL-15, a macrophage (CD68)-derived TCGF, can mediate early rejection episodes in BT563 mAb (murine anti-human CD25)-treated cardiac transplant patients. The report represents a logical extension of the previous work by this group that has already documented intragraft expression of IL-15 mRNA in the absence of IL-2 mRNA production after clinical cardiac transplantation (5). The present study is important because unlike previous “nonfunctional” reverse transcriptase-polymerase chain reaction/immunohistochemical-based analyses, it evaluates, for the first time, the actual biological activity of IL-15 on CD25-blocked cells. In addition, it exemplifies how our findings from well-defined in vitro and in vivo animal transplantation models may be relevant and applicable in the clinical setting. Peripheral blood samples from patients undergoing biopsy-proven cardiac rejection despite a mixture of immunosuppressive therapy that included anti-CD25 mAb, were analyzed for the frequency of IL-2 and IL-15–proliferative T cells. Two interesting observations in this study highlight the complexity of the IL-2/IL-2R signaling pathway. First, anti-CD25 therapy significantly impaired but did not abolish subsequent in vitro responsiveness to both recombinant human IL-2 and IL-15. Second, although the addition of exogenous anti-CD25 mAb did block the bulk of the residual IL-2 driven responsiveness, it did not affect whatsoever IL-15–driven T-cell proliferation. Hence, when the IL-2/IL-2R pathway is affected, IL-15 can mediate T-cell responses in the immune rejection cascade. In other words, growth signals provided by shared IL-2R β and γ chains, and by IL-15–specific binding protein (IL-15R α) (Fig. 1B), can still induce allograft loss despite concomitant blockage of the high-affinity IL-2R complex. How difficult indeed it is to “cheat” nature! Clearly, the widespread distribution of IL-15 α as well as of IL-2/15 β and common γ receptor subunits represent one of the possible mechanisms underlying the pleiotropy of IL-2 and IL-15. Moreover, by sharing common chain receptors, IL-2 and IL-15 have been shown to activate identical transcription Stat 3 and 5 proteins, which in turn may also contribute to the generation of redundant cytokine actions (6). Indeed, IL-2 and IL-15 have a number of redundant functions such as induction of T-cell activation/T cytotoxic effector functions, T-cell chemoattraction, costimulation of Ig synthesis by B cells, and activation of monocytes. To further dissect the pleiotropic and redundant functions of cytokines sharing common chain receptors, we need to further investigate the downstream events of these effector molecules associated with the receptors. Unlike IL-2, however, IL-15 transcripts are not usually detected in normal or activated T cells. Moreover, IL-15 seems pivotal in the differentiation of natural killer cells, and it may exert some unique functions on nonlymphoid cells, including skeletal muscle cells, microglia, and mast cells. The generation of IL-15 knockout mouse should assist in the definition of nonredundant IL-15 functions. What is the future for IL-2R–targeted therapy in organ transplantation? The report of Baan et al. clearly demonstrates that adjunctive inhibition of the common receptor components of TCGFs may be of utmost importance. A number of possibilities can be envisioned for such a strategy, including the development of an inhibitor of Jak3, the signaling molecule utilized by both IL-2 and IL-15, as an agent to yield controlled immunosuppression. In addition, in a recent study, selective targeting of the IL-15R with a novel long-lived IL-15 mutant Fcγ2a protein decreased leukocyte infiltration and ameliorated antigen-specific delayed-type hypersensitivity responses in vivo (7). Such studies and the pioneering work of Baan et al. pave the way for the development of new clinical trials that will further exploit the therapeutic potential of novel modalities generated from lessons learned on cytokine pleiotropy and redundancy. The application of agents that target IL-15/IL-15R and signaling elements shared by IL-2 and IL-15 should provide new perspectives for the treatment of T helper 1-dependent disease processes, including organ allograft rejection.