Prolonged xenograft survival induced by inducible costimulator-Ig is associated with increased forkhead box P3(+) cells

Inducible Co-Stimulator (ICOS) in transplantation: A review

Prevention of T cell activation is one of the goals of successful organ and tissue transplantation. Blockade of T cell co-stimulation, particularly of the CD28:B7 interaction, has been shown to prolong graft survival. Inducible Co-Stimulator (ICOS) is the third member of the B7 family and here we review the literature on ICOS, its receptor (B7RP-1), and blockade of this pathway in transplant models. ICOS:B7RP-1 are a single receptor:ligand pair with a loss of function of either being implicated in some autoimmune diseases. ICOS has multiple functions, related to its constitutive expression on B cells and activated T cells. In in vitro transplant models, ICOS:B7RP-1 blockade has produced mixed results as to its ability to modulate lymphocyte proliferation. Several in vivo transplant models demonstrate varying degrees of success in prolonging graft survival. Timing and dose of treatment appear important, and combination with other immunosuppressive treatments may also be of benefit. As ICOS has multiple functions, it may be that the observed variable results are due to inadvertent inactivation of graft protective functions. If these barriers can be overcome, ICOS:B7RP-1 blockade could provide an important target for future immunosuppression regimens.

Antigen-specific CD4+ CD25+ T cells induced by locally expressed ICOS-Ig: the role of Foxp3, Perforin, Granzyme B and IL-10 - an experimental study

We have previously reported that ICOS-Ig expressed locally by a PIEC xenograft induces a perigraft cellular accumulation of CD4⁺ CD25⁺ Foxp3⁺ T cells and specific xenograft prolongation. In the present study we isolated and purified CD4⁺ CD25⁺ T cells from ICOS-Ig secreting PIEC grafts to examine their phenotype and mechanism of xenograft survival using knockout and mutant mice. CD4⁺ CD25⁺ T cells isolated from xenografts secreting ICOS-Ig were analysed by flow cytometry and gene expression by real-time PCR. Regulatory function was examined by suppression of xenogeneic or allogeneic primed CD4 T cells in vivo. Graft prolongation was shown to be dependent on a pre-existing Foxp3⁺ Treg, IL-10, perforin and granzyme B. CD4⁺ CD25⁺ Foxp3⁺ T cells isolated from xenografts secreting ICOS-Ig demonstrated a phenotype consistent with nTreg but with a higher expression of CD275 (ICOSL), expression of CD278 (ICOS) and MHC II and loss of CD73. Moreover, these cells were functional and specifically suppressed xenogeinic but not allogeneic primed T cells in vivo.

Potential targeting of B7-H4 for the treatment of cancer

Observations noting the presence of white blood cell infiltrates within tumors date back more than a century, however the cellular and molecular mechanisms regulating tumor immunity continue to be elucidated. The recent successful use of monoclonal antibodies to block immune regulatory pathways to enhance tumor-specific immune responses for the treatment of cancer has encouraged the identification of additional immune regulatory receptor/ligand pathways. Over the past several years, a growing body of data has identified B7-H4 (VTCN1/B7x/B7S1) as a potential therapeutic target for the treatment of cancer. The potential clinical significance of B7-H4 is supported by the high levels of B7-H4 expression found in numerous tumor tissues and correlation of the level of expression on tumor cells with adverse clinical and pathologic features, including tumor aggressiveness. The biological activity of B7-H4 has been associated with decreased inflammatory CD4⁺ T-cell responses and a correlation between B7-H4-expressing tumor-associated macrophages and FoxP3⁺ regulatory T cells (Tregs) within the tumor microenvironment. Since B7-H4 is expressed on tumor cells and tumor-associated macrophages in various cancer types, therapeutic blockade of B7-H4 could favorably alter the tumor microenvironment allowing for antigen-specific clearance tumor cells. The present review highlights the therapeutic potential of targeting B7-H4.

Immunological challenges and therapies in xenotransplantation

Xenotransplantation, or the transplantation of cells, tissues, or organs between different species, was proposed a long time ago as a possible solution to the worldwide shortage of human organs and tissues for transplantation. In this setting, the pig is currently seen as the most likely candidate species. In the last decade, progress in this field has been remarkable and includes a better insight into the immunological mechanisms underlying the rejection process. Several immunological hurdles nonetheless remain, such as the strong antibody-mediated and innate or adaptive cellular immune responses linked to coagulation derangements, precluding indefinite xenograft survival. This article reviews our current understanding of the immunological mechanisms involved in xenograft rejection and the potential strategies that may enable xenotransplantation to become a clinical reality in the not-too-distant future.

ICOS regulates the generation and function of human CD4+ Treg in a CTLA-4 dependent manner

Inducible co-stimulator (ICOS) is a member of CD28/Cytotoxic T-lymphocyte Antigen-4 (CTLA-4) family and broadly expressed in activated CD4(+) T cells and induced regulatory CD4(+) T cells (CD4(+) iTreg). ICOS-related signal pathway could be activated by the interaction between ICOS and its ligand (ICOSL). In our previous work, we established a cost-effective system to generate a novel human allo-antigen specific CD4(hi) Treg by co-culturing their naïve precursors with allogeneic CD40-activated B cells in vitro. Here we investigate the role of ICOS in the generation and function of CD4(hi) Treg by interrupting ICOS-ICOSL interaction with ICOS-Ig. It is found that blockade of ICOS-ICOSL interaction impairs the induction and expansion of CD4(hi) Treg induced by allogeneic CD40-activated B cells. More importantly, CD4(hi) Treg induced with the addition of ICOS-Ig exhibits decreased suppressive capacity on alloantigen-specific responses. Dysfunction of CD4(hi) Treg induced with ICOS-Ig is accompanied with its decreased exocytosis and surface CTLA-4 expression. Through inhibiting endocytosis with E64 and pepstatin A, surface CTLA-4 expression and suppressive functions of induced CD4(hi) Treg could be partly reversed. Conclusively, our results demonstrate the beneficial role of ICOS-ICOSL signal pathway in the generation and function of CD4(hi) Treg and uncover a novel relationship between ICOS and CTLA-4.

Targeting the B7 family of co-stimulatory molecules: successes and challenges

As more patient data is cross-referenced with animal models of disease, the primary focus on T(h)1 autoreactive effector cell function in autoimmune diseases, such as rheumatoid arthritis and multiple sclerosis, has shifted towards the role of T(h)17 autoreactive effector cells and the ability of regulatory T cells (T(reg)) to modulate the pro-inflammatory autoimmune response. Therefore, the currently favored hypothesis is that a delicate balance between T(h)1/17 effector cells and T(reg) cell function is critical in the regulation of inflammatory autoimmune disease. An intensive area of research with regard to the T(h)1/17:T(reg) cell balance is the utilization of blockade and/or ligation of various co-stimulatory or co-inhibitory molecules, respectively, during ongoing disease to skew the immune response toward a more tolerogenic/regulatory state. Currently, FDA-approved therapies for multiple sclerosis patients are all aimed at the suppression of immune cell function. The other favored method of treatment is a modulation or deletion of autoreactive immune cells via short-term blockade of activating co-stimulatory receptors via treatment with fusion proteins such as CTLA4-Ig and CTLA4-FasL. Based on the initial success of CTLA4-Ig, there are additional fusion proteins that are currently under development. Examples of the more recently identified B7/CD28 family members are PD-L1, PD-L2, inducible co-stimulatory molecule-ligand (ICOS-L), B7-H3, and B7-H4, all of which may emerge as potential fusion protein therapeutics, each with unique, yet often overlapping functions. The expression of both stimulatory and inhibitory B7 molecules seems to play an essential role in modulating immune cell function through a variety of mechanisms, which is supported by findings that suggest each B7 molecule has developed its own indispensable niche in the immune system. As more data are generated, the diagnostic and therapeutic potential of the above B7 family-member-derived fusion proteins becomes ever more apparent. Besides defining the biology of these B7/CD28 family members in vivo, additional difficulty in the development of these therapies lies in maintaining the normal immune functions of recognition and reaction to non-self-antigens following viral or bacterial infection in the patient. Further complicating the clinical translation of these therapies, the mechanism of action identified for a particular reagent may depend upon the method of immune-cell activation and the subset of immune cells targeted in the study.

Potential of T-regulatory cells to protect xenografts

PURPOSE OF REVIEW

Immunological barriers still preclude clinical xenotransplantation. The protective role of CD4(+)CD25(+)Foxp3(+) T-regulatory cells (Treg) in allotransplantation is well described and, therefore, could represent a promising therapeutical tool for xenotransplantation. This review addresses the latest findings on Treg in xenotransplantation research.

RECENT FINDINGS

In vivo, costimulation blockade-based strategies including anti-CD154 monoclonal antibodies (mAbs) in combination with rapamycin or anti-LFA-1 mAb prolonged both concordant and discordant islets xenografts survival in a Treg-dependent manner. In vitro, IL-10 secretion was shown to be critical for the suppression of xenogeneic responses mediated by Treg. Moreover, transgenic expression of inducible costimulator-immunoglobulin or PD-L1 on porcine endothelial cells inhibited human T-cell proliferation in vitro and was associated with the induction of Treg and IL-10 secretion. CXCR3 mediated the recruitment of Treg to pig endothelium. Finally, the recruitment of human Treg was enhanced by the immobilization of human CCL17 on pig endothelium.

SUMMARY

There is increasing evidence for the potential of CD4(+)CD25(+)Foxp3(+) Treg to protect xenografts. Induction of Treg in recipients and/or recruitment of human Treg to pig endothelium may represent novel strategies to prevent cell-mediated rejection in pig-to-human xenotransplantation.

CD28 family and chronic rejection: "to belatacept...And beyond!"

Kidneys are one of the most frequently transplanted human organs. Immunosuppressive agents may prevent or reverse most acute rejection episodes; however, the graft may still succumb to chronic rejection. The immunological response involved in the chronic rejection process depends on both innate and adaptive immune response. T lymphocytes have a pivotal role in chronic rejection in adaptive immune response. Meanwhile, we aim to present a general overview on the state-of-the-art knowledge of the strategies used for manipulating the lymphocyte activation mechanisms involved in allografts, with emphasis on T-lymphocyte costimulatory and coinhibitory molecules of the B7-CD28 superfamily. A deeper understanding of the structure and function of these molecules improves both the knowledge of the immune system itself and their potential action as rejection inducers or tolerance promoters. In this context, the central role played by CD28 family, especially the relationship between CD28 and CTLA-4, becomes an interesting target for the development of immune-based therapies aiming to increase the survival rate of allografts and to decrease autoimmune phenomena. Good results obtained by the recent development of abatacept and belatacept with potential clinical use aroused better expectations concerning the outcome of transplanted patients.

Current progress in xenogeneic tolerance

PURPOSE OF REVIEW

The present review updates the current status of research regarding the immunologic responses of the recipient following xenotransplantation. Additionally, we present the recent progress with attempts to induce xenogeneic tolerance induction.

RECENT FINDINGS

There continues to be great interest in xenotransplantation. Recently, descriptions of the mechanisms responsible for attempted T-cell xenogeneic tolerance in both large and small animal models have improved xenogeneic graft survivals. Additionally, the cellular signaling mechanisms, such as those involving CD39, CD44, and CD47, are proving to be highly important. Using the mixed chimerism approach to tolerance in xenogeneic model may be encouraging, especially given the recent clarification of the role for macrophage-induced phagocytosis of xenogeneic donor cells.

SUMMARY

Induction of tolerance to xenogeneic antigens has been accomplished only in small animals; however, graft survivals in large animal models continue to improve. Further clarification of both the adaptive and innate immune responses to xenogeneic antigens is required for success to continue.