Demonstration of local immunosuppression with methylprednisolone in the sponge matrix allograft model

Emerging local immunomodulatory strategies to circumvent systemic immunosuppression in cell transplantation

INTRODUCTION

Cell transplantation is a promising curative therapeutic strategy whereby impaired organ functions can be restored without the need for whole organ transplantation. A key challenge in allotransplantation is the requirement for life-long systemic immunosuppression to prevent rejection, which is associated with serious adverse effects such as increased risk of opportunistic infections and the development of neoplasms. This challenge underscores the urgent need for novel strategies to prevent graft rejection while abrogating toxicity-associated adverse events.

AREAS COVERED

We review recent advances in immunoengineering strategies for localized immunomodulation that aim to support allograft function and provide immune tolerance in a safe and effective manner.

EXPERT OPINION

Immunoengineering strategies are tailored approaches for achieving immunomodulation of the transplant microenvironment. Biomaterials can be adapted for localized and controlled release of immunomodulatory agents, decreasing the effective dose threshold and frequency of administration. The future of transplant rejection management lies in the shift from systemic to local immunomodulation with suppression of effector and activation of regulatory T cells, to promote immune tolerance.

Suppression of cytotoxic T lymphocyte activity by 2,3,7,8-tetrachlorodibenzo-p-dioxin occurs in vivo, but not in vitro, and is independent of corticosterone elevation

Previous studies have shown that 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) is a potent immunosuppressive compound. In our laboratory, TCDD and structurally related polychlorinated biphenyls (PCBs) have been shown to suppress alloantigen-specific cytotoxic T lymphocyte (CTL) activity in C57B1/6 mice. PCB-induced CTL suppression occurs coincident with significant elevation of plasma glucocorticoid (GC) levels (> 500 ng/ml). Since GC elevation can cause immune suppression, this study was conducted to determine if TCDD-induced CTL suppression is correlated with elevation of plasma corticosterone (CS), the major GC in mice. Single oral doses of TCDD (2.5-40 micrograms/kg) induced a dose-dependent suppression of CTL activity with a calculated 50% immunosuppressive dose (ID50) occurring at 7.2 micrograms/kg. When total lytic units (LU)/spleen were calculated, the ID50 was 2.8 micrograms/kg. In contrast, plasma CS levels were not significantly altered at doses below 40 micrograms/kg. These data suggest that TCDD-induced CTL suppression is not dependent on CS elevation. The direct effect of TCDD on CTL generation was tested by adding TCDD at 10(-13)-10(-9) M to in vitro mixed lymphocyte-tumor cell (MLTC) cultures. No alteration of CTL activity was observed after 5 days of culture at any TCDD concentration. In contrast, CS alone significantly suppressed CTL activity in vitro. CS-induced CTL suppression in vitro was neither enhanced nor inhibited by the presence of TCDD. These results suggest that TCDD causes CTL suppression in vivo by a mechanism that does not involve CS.

Inhibition of rat splenocyte proliferation with methylprednisolone: in vivo effect of liposomal formulation

The effect of a liposomal formulation of methylprednisolone (MPL) on the inhibition of lymphocyte proliferation in spleen cells was investigated following IV dosing in rats. Liposomes do not alter the suppressive action of MPL when placed in lymphocyte culture. Rat splenocytes were found to have greater sensitivity to MPL (EC50 = 7.9 nM) than do human peripheral blood lymphocytes (EC50 = 28 nM). In vivo studies in rats utilized 2 mg/kg IV bolus doses of liposomal MPL compared to drug in solution. Animals were sacrificed at various times post-dosing until 120 h, spleen was excised and, after incubation of lymphocytes with PHA, splenocyte blastogenic responses were assessed by measuring cellular incorporation of 3H-thymidine. The suppressive effect of liposomal MPL in comparison with free drug was significantly prolonged (> 120 h vs < 18 h). Inhibition effects versus time were described by a pharmacodynamic model using MPL concentrations in plasma as an input function. A nonlinear relationship was found between suppression of splenocyte proliferation and the concentration of bound glucocorticoid receptors in spleen. Only partial receptor occupancy accompanied complete lymphocyte suppression. The suppression of endogenous corticosterone in plasma for both treatments was similar with values from L-MPL rats returning to baseline after 24 h. These results demonstrate enhanced efficacy of local immunosuppression by targeting spleen with liposomal MPL.

An improved model for the examination of biological effects of locally administered cytokines

Cytokines incorporated into agarose blocks and implanted subcutaneously into mice establish an in vivo gradient which can be used to mimic a local inflammatory process. We have developed a model in which cellular influx into cytokine impregnated blocks parallels the normal cellular reaction to infections or wounds. Agarose blocks containing supernatants from ConA activated rat spleen cells attracted neutrophils within 4 h. These cells were followed by lymphocytes and macrophages in 24 h. Flow cytometry analysis of lymphoid cells on day 1 revealed that 38% were Ig+ (B cell marker), 60% MAC-2,3+ and 20% Thy 1.2+ of which only a small fraction were expressing CD4 on their surface. These numbers changed with time following implantation of the blocks. Initially, isolated adherent cells (macrophages) were resting, with low phagocytic activity. Cells isolated from blocks at later time points were activated, as evidenced by their increased ability to ingest fluoresceinated beads. The secretion patterns of cells trafficking to murine rIL-1 containing agarose blocks were examined. TNF, IL-6 and antibody secreting cells were found. No IL-2 was detected at any time. We believe that this model will be of value in studies of local actions of cytokines.