Cellular Immunobiology and Molecular Mechanisms in Alloimmunity-Pathways of Immunosuppression

Current maintenance immunosuppression commonly comprises a synergistic combination of tacrolimus as calcineurin inhibitor (CNI), mycophenolic acid, and glucocorticoids. Therapy is often individualized by steroid withdrawal or addition of belatacept or inhibitors of the mechanistic target of rapamycin. This review provides a comprehensive overview of their mode of action, focusing on the cellular immune system. The main pharmacological action of CNIs is suppression of the interleukin-2 pathway that leads to inhibition of T cell activation. Mycophenolic acid inhibits the purine pathway and subsequently diminishes T and B cell proliferation but also exerts a variety of effects on almost all immune cells, including inhibition of plasma cell activity. Glucocorticoids exert complex regulation via genomic and nongenomic mechanisms, acting mainly by downregulating proinflammatory cytokine signatures and cell signaling. Belatacept is potent in inhibiting B/T cell interaction, preventing formation of antibodies; however, it lacks the potency of CNIs in preventing T cell-mediated rejections. Mechanistic target of rapamycin inhibitors have strong antiproliferative activity on all cell types interfering with multiple metabolic pathways, partly explaining poor tolerability, whereas their superior effector T cell function might explain their benefits in the case of viral infections. Over the past decades, clinical and experimental studies provided a good overview on the underlying mechanisms of immunosuppressants. However, more data are needed to delineate the interaction between innate and adaptive immunity to better achieve tolerance and control of rejection. A better and more comprehensive understanding of the mechanistic reasons for failure of immunosuppressants, including individual risk/benefit assessments, may permit improved patient stratification.

In or out of control: Modulating regulatory T cell homeostasis and function with immune checkpoint pathways

Regulatory T cells (Tregs) are the master regulators of immunity and they have been implicated in different disease states such as infection, autoimmunity and cancer. Since their discovery, many studies have focused on understanding Treg development, differentiation, and function. While there are many players in the generation and function of truly suppressive Tregs, the role of checkpoint pathways in these processes have been studied extensively. In this paper, we systematically review the role of different checkpoint pathways in Treg homeostasis and function. We describe how co-stimulatory and co-inhibitory pathways modulate Treg homeostasis and function and highlight data from mouse and human studies. Multiple checkpoint pathways are being targeted in cancer and autoimmunity; therefore, we share insights from the clinic and discuss the effect of experimental and approved therapeutics on Treg biology.

Functional analysis of acquired CD28 mutations identified in cutaneous T cell lymphoma

CD28 is the major costimulatory receptor on T cells regulating proliferation, survival and effector function. Acquired mutations in the extracellular domain of CD28 have been identified in patients with cutaneous T cell lymphoma, angioimmunoblastic T cell lymphoma and other T cell neoplasms, suggesting it may contribute to disease pathogenesis. We used a heterologous system in which mutant human CD28 was expressed on primary murine T cells deficient in CD28 to ascertain how specific mutations identified in a genetic screen of patients with cutaneous T cell lymphoma affected normal T cell function. All three mutant CD28 proteins examined enhanced CD28-dependent T cell proliferation and effector function. These data suggest that the mutant CD28 isoforms could accelerate tumor cell growth and increase tumor burden in affected patients. Interruption of CD28:ligand interactions may be an effective, targeted therapy for a subset of patients whose tumors bear the mutant CD28 receptor.

Steroid withdrawal in kidney allograft recipients

This review highlights the aggregate of knowledge obtained from the temporal trend of kidney transplant immune suppression. We will discuss the burden of steroid side effects and their impact on quality of life in kidney allograft recipients, which have led to minimizing steroid exposure. Issues arising since the inception of the concept of steroid withdrawal will be discussed, along with how they have continually led to a shift in research focus on this subject matter. The usefulness of surveillance biopsies and how further elucidation of the pathophysiology of interstitial fibrosis and tubular atrophy could contribute to improving long-term allograft outcomes will also be discussed. We will elaborate on the role of calcineurin inhibitor minimization alongside steroid withdrawal in improving long-term graft survival. Future expectations of subsequent studies with a view to improving overall kidney allograft outcomes by eliminating attendant problems associated with steroids will also be covered.

Calcineurin inhibitors: 40 years later, can't live without

Calcineurin inhibitors (CNIs) revolutionized the field of organ transplantation and remain the standard of care 40 years after the discovery of cyclosporine. The early impressive results of cyclosporine in kidney transplant recipients led to its subsequent use in other organ transplant recipients and for treatment of a variety of autoimmune diseases as well. In this review, we examine the discovery of CNIs, their mechanism of action, preclinical and clinical studies with CNIs, and the usage of CNIs in nontransplant recipients. We review the mechanisms of renal toxicity associated with CNIs and the recent efforts to avoid or reduce usage of these drugs. Although minimization strategies are possible, safe, and of potential long-term benefit, complete avoidance of CNIs has proven to be more challenging than initially thought.