Evidence that a similar range of alloimmune responses can develop in murine and human allograft recipients

Crucial role of corneal lymphangiogenesis for allograft rejection in alkali-burned cornea bed

BACKGROUND

To examine the time course of hemangiogenesis, lymphangiogenesis, inflammation after corneal alkaline burns and compare with the importance of corneal hemangiogenesis, lymphangiogenesis and inflammation in allograft rejection on alkali-burned cornea bed, respectively.

METHODS

Rat corneal hemangiogenesis and lymphangiogenesis were examined by whole mount immunofluorescence and double enzyme-histochemistry, and the state of corneal inflammation was evaluated by inflammation index scoring and histopathology. Then, corneal transplantations were divided into six groups and performed before the burn (group A) and on day 3 (group B), 2 weeks (group C), 5 weeks (group D), 6 weeks (group E) and 8 weeks (group F) after alkaline burns, respectively. The immune rejection of grafts was evaluated by interferon-gamma, interleukin-2 enzyme-linked immunosorbent assay and slit-lamp examination.

RESULTS

Both corneal lymphatic and blood vessels reached the top 2 weeks after the burn. Corneal lymphangiogenesis disappeared 5 weeks after the burn, and corneal hemangiogenesis regressed completely 3 weeks later. Corneal inflammation was strong on day 3, but resolved 6 weeks after the burn. Compared with other groups, the mean survival time of groups B (4.67 +/- 1.03 days) and C (5.00 +/- 0.63 days) was significantly shorter (P < 0.05). The difference of mean survival time of grafts between group D (9.50 +/- 1.05 days) and group E (9.83 +/- 0.75 days), between group D and group F (10.00 +/- 0.89 days) was not significant (P > 0.05).

CONCLUSIONS

Corneal lymphangiogenesis presents for a shorter duration than corneal hemangiogenesis or corneal inflammation but plays a crucial role in allograft rejection on alkali-burned cornea bed.

Disruption of murine cardiac allograft acceptance by latent cytomegalovirus

Cytomegalovirus (CMV) reactivation is a well-described complication of solid organ transplantation. These studies were performed to (1) determine if cardiac allograft transplantation of latently infected recipients results in reactivation of CMV and (2) determine what impact CMV might have on development of graft acceptance/tolerance. BALB/c cardiac allografts were transplanted into C57BL/6 mice with/without latent murine CMV (MCMV). Recipients were treated with gallium nitrate induction and monitored for graft survival, viral immunity and donor reactive DTH responses. Latently infected allograft recipients had approximately 80% graft loss by 100 days after transplant, compared with approximately 8% graft loss in naïve recipients. PCR evaluation demonstrated that MCMV was transmitted to cardiac grafts in all latently infected recipients, and 4/8 allografts had active viral transcription compared to 0/6 isografts. Latently infected allograft recipients showed intragraft IFN-alpha expression consistent with MCMV reactivation, but MCMV did not appear to negatively influence regulatory gene expression. Infected allograft recipients had disruption of splenocyte DTH regulation, but recipient splenocytes remained unresponsive to donor antigen even after allograft losses. These data suggest that transplantation in an environment of latent CMV infection may reactivate virus, and that intragraft responses disrupt development of allograft acceptance.

Spontaneous renal allograft acceptance associated with "regulatory" dendritic cells and IDO

MHC-mismatched DBA/2 renal allografts are spontaneously accepted by C57BL/6 mice by poorly understood mechanisms, but both immune regulation and graft acceptance develop without exogenous immune modulation. Previous studies have shown that this model of spontaneous renal allograft acceptance is associated with TGF-beta-dependent immune regulation, suggesting a role for T regulatory cells. The current study shows that TGF-beta immune regulation develops 30 days posttransplant, but is lost by 150 days posttransplant. Despite loss of detectable TGF-beta immune regulation, renal allografts continue to function normally for >200 days posttransplantation. Because of its recently described immunoregulatory capabilities, we studied IDO expression in this model, and found that intragraft IDO gene expression progressively increases over time, and that IDO in "regulatory" dendritic cells (RDC) may contribute to regulation associated with long-term maintenance of renal allografts. Immunohistochemistry evaluation confirms the presence of both Foxp3+ T cells and IDO+ DCs in accepted renal allografts, and localization of both cell types within accepted allografts suggests the possibility of synergistic involvement in allograft acceptance. Interestingly, at the time when RDCs become detectable in spleens of allograft acceptors, approximately 30% of these mice challenged with donor-matched skin allografts accept these skin grafts, demonstrating progression to "true" tolerance. Together, these data suggest that spontaneous renal allograft acceptance evolves through a series of transient mechanisms, beginning with TGF-beta and T regulatory cells, which together may stimulate development of more robust regulation associated with RDC and IDO.

In vivo treatment of mice with heat shock protein, gp 96, improves survival of skin grafts with minor and major antigenic disparity

Immunization with high doses of heat shock protein, gp 6 given in vivo, has been shown to mediate the activation of CD4+ T cell, which in turn suppresses an ongoing CD8+ immune response. Here we demonstrate that high doses of gp 6 (HDgp 6) (100 microg given subcutaneously on days 0 and 7 following skin graft transplantation) improve survival of skin grafts with both minor and major histocompatibility. First, skin from male C57BL/6 donors was grafted onto female C57BL/6 recipients that were subsequently treated with either HDgp 6 or buffer and graft survival was monitored. At 64 days post-transplantation, graft recipients treated with HDgp 6 showed 56% graft survival compared to 10% in those treated with buffer. Further, HDgp 6 down regulates a secondary immune response as well. Skin graft obtained from a male C57BL/6 donor was transplanted to a female C57BL/6 recipient that had previously rejected a male skin graft. Recipients that were treated with HDgp 96 showed a 75% graft survival of the secondary graft at 64 days post-transplantation, whereas recipients treated with buffer rejected their secondary grafts within 37 days. Finally, the effect of HDgp 96 treatment on survival of graft with major histoincompatibility was tested. Skin grafts from female BALB/c donors were transplanted to female C57BL/6 recipients previously treated with either HDgp 96 or buffer. Although all the grafts were ultimately rejected, mice treated with HDgp96 showed a delay in the rejection as compared with buffer controls. Our findings indicate that HDgp 96 improves survival of grafts with both minor and major antigenic disparity and could lead to developing novel therapies in transplant medicine.

A renewable source of donor cells for repetitive monitoring of T- and B-cell alloreactivity

A major impediment to repetitive monitoring of alloreactivity or tolerance is the limited supply of donor cells available for assays of host-versus-graft T- and B-cell reactivity. In this paper, we describe the use of CD40L stimulated CD19(+) B cells as targets or stimulators in flow cytometric crossmatching (FXM), mixed lymphocyte reactivity and IFN-gamma ELISPOT assays. Stimulated B cells (sBc) express high levels of MHC class I and II, as well as the costimulatory molecules CD80 and CD86. They can be polyclonally expanded and frozen for later use. We describe the use of sBc in ELISPOT, mixed lymphocyte cultures and FXM. CD4(+) T cells exposed to sBc express a similar cytokine profile as those stimulated with unfractionated PBMC. We further analyzed T- and B-cell responses in 14 patients on the renal transplant waiting list, finding that those with an elevated panel reactive antibody (PRA) (>60%) had higher alloreactive T-cell precursor frequencies as measured by CDFSE MLR and IFN-gamma ELISPOT. We conclude that sBc are a renewable source of donor-specific target/stimulator cells for use in repetitive and coordinate assays of B- and T-cell alloreactivity.

Immunologic monitoring

The development of reliable in vitro assays that could allow the quantitation and characterization of anti-donor alloimmune responses has always been a goal in clinical transplantation, both to predict presensitization to the transplanted tissue and to be able to identify rejection without resorting to more invasive tests. With recent development in our understanding of transplantation biology and therapeutics, there is a real expectation that these tests may be used to identify tolerance as much as to predict rejection. The traditional limiting dilution assays still have a contribution to make and are being complemented by an array of tools, such as ELISpot, flow cytometry-based techniques, and microarray analysis. The assays that have been informative, to date, are discussed in this review. This information will lead, at least, to a better understanding of how and when the rejection process occurs. More interestingly, the objective is to apply this information to evaluate tolerance-inducing strategies or to identify patients that have become tolerant to their graft and can be weaned of immunosuppression. Of course sensitive, accurate and specific immunologic monitoring has applications well beyond the field of transplantation.