CCR2 regulates monocyte recruitment as well as CD4 T1 allorecognition after lung transplantation

Graft rejection remains a formidable problem contributing to poor outcomes after lung transplantation. Blocking chemokine pathways have yielded promising results in some organ transplant systems. Previous clinical studies have demonstrated upregulation of CCR2 ligands following lung transplantation. Moreover, lung injury is attenuated in CCR2-deficient mice in several inflammatory models. In this study, we examined the role of CCR2 in monocyte recruitment and alloimmune responses in a mouse model of vascularized orthotopic lung transplantation. The CCR2 ligand MCP-1 is upregulated in serum and allografts following lung transplantation. CCR2 is critical for the mobilization of monocytes from the bone marrow into the bloodstream and for the accumulation of CD11c(+) cells within lung allografts. A portion of graft-infiltrating recipient CD11c(+) cells expresses both recipient and donor MHC molecules. Two-photon imaging demonstrates that recipient CD11c(+) cells are associated with recipient T cells within the graft. While recipient CCR2 deficiency does not prevent acute lung rejection and is associated with increased graft infiltration by T cells, it significantly reduces CD4(+) T(h)1 indirect and direct allorecognition. Thus, CCR2 may be a potential target to attenuate alloimmune responses after lung transplantation.

Molecular mechanisms of allorecognition in a basal chordate

Allorecognition has been described in many metazoan phyla, from the sponges to the mammals. In vertebrates, allorecognition is a result of a MHC-based recognition event central to adaptive immunity. However, the origin of the adaptive immune system and the potential relationship to more primitive allorecognition systems is unclear. The colonial ascidian, Botryllus schlosseri, has been used as a model organism for the study of allorecognition for over a century, as it undergoes a natural transplantation reaction controlled by a single, highly polymorphic locus. Herein we will summarize our current understanding of the molecular mechanisms that underlie this innate allorecognition reaction.

Ex vivo inhibition of NF-kappaB signaling in alloreactive T-cells prevents graft-versus-host disease

The ex vivo induction of alloantigen-specific hyporesponsiveness by costimulatory pathway blockade or exposure to immunoregulatory cytokines has been shown to inhibit proliferation, IL-2 production, and the graft-versus-host disease (GVHD) capacity of adoptively transferred T-cells. We hypothesized that inhibition of the intracellular NF-kappaB pathway in alloreactive T-cells, which is critical for T-cell activation events including IL-2 transcription, could lead to alloantigen hyporesponsiveness and loss of GVHD capacity. We demonstrate that treatment of mixed lymphocyte reaction (MLR) cultures with PS1145, a potent inhibitor of NF-kappaB activation, can induce T-cell hyporesponsiveness to alloantigen in primary and secondary responses while preserving in vitro responses to potent mitogenic stimulation. GVHD lethality in recipients of ex vivo PS1145-treated cells was profoundly inhibited. Parking of control or PS1145-treated MLR cells in syngeneic Rag(-/-) recipients resulted in intact contact hypersensitivity (CHS) responses. However, GVHD lethality capacity also was restored, suggesting that lymphopenic expansion uncoupled alloantigen hyporesponsiveness. These results indicate that the NF-kappaB pathway is a critical regulator of alloresponses and provide a novel small molecule inhibitor based approach that is effective in preventing early posttransplant GVHD lethality but that also permits donor T-cell responses to recover after a period of lymphopenic expansion.

Allorecognition and chimerism in an invertebrate model organism

The presence of highly specific histocompatibility reactions in colonial marine invertebrates that lack adaptive immune systems (such as the sponges, cnidarians, bryozoans and ascidians) provides a unique opportunity to investigate the evolutionary roots of allorecognition and to explore whether homologous innate recognition systems exist in vertebrates. Conspecific interactions among adult animals in these groups are regulated by highly specific allorecognition systems that restrict somatic fusion to self or close kin. In Hydractinia (Cnidaria:Hydrozoa), fusion/rejection responses are controlled by two linked genetic loci. Alleles at each locus are co-dominantly inherited. Colonies fuse if they share at least one haplotype, reject if they share no haplotypes, and display transitory fusion if they share only one allele in a haplotype-a pattern that echoes natural killer cell responses in mice and humans. Allorecognition in Hydractinia and other marine invertebrates serves as a safeguard against stem cell or germline parasitism thus, limiting chimerism to closely related individuals. These animals fail to become tolerant even if exposed during early development to cells from a histoincompatible individual. Detailed analysis of the structure and function of molecules responsible for allorecognition in basal marine invertebrates could provide clues to the innate mechanisms by which higher animals respond to organ and cell allografts, including embryonic tissues.

Donor-reactive CD8 memory T cells infiltrate cardiac allografts within 24-h posttransplant in naive recipients

Normal immune responses stimulated by pathogenic and environmental antigens generate memory T cells that react with donor antigens and no currently used immunosuppressive drug completely inhibits memory T-cell function. While donor-reactive memory T cells clearly compromise graft outcomes, mechanisms utilized by memory T cells to promote rejection are largely unknown. In this study, we investigated how early endogenous memory cells infiltrate and express effector function in cardiac allografts. Endogenous CD8 memory T cells in nonsensitized recipients distinguish syngeneic versus allogeneic cardiac allografts within 24 h of reperfusion. CD8-dependent production of IFN-gamma and CXCL9/Mig was observed 24 to 72 h posttransplant in allografts but not isografts. CXCL9 was produced by donor cells in response to IFN-gamma made by recipient CD8 T cells reactive to donor class I major histocompatibility complex (MHC) molecules. Activated CD8 T cells were detected in allografts at least 3 days before donor-specific effector T cells producing IFN-gamma were detected in the recipient spleen. Early inflammation mediated by donor-reactive CD8 memory T cells greatly enhanced primed effector T-cell infiltration into allografts. These results suggest that strategies for optimal inhibition of alloimmunity should include neutralization of infiltrating CD8 memory T cells within a very narrow window after transplantation.

Embryonic chimerism does not induce tolerance in an invertebrate model organism

In colonial marine invertebrates, allorecognition restricts somatic fusion and thus, chimerism, to histocompatible individuals. Little is understood, however, about how invertebrates respond to chimerism formed across histocompatibility barriers or whether embryonic exposure to histoincompatible cells induces allotolerance. We here evaded natural allorecognition barriers by generating well mixed embryonic chimeras of Hydractinia symbiolongicarpus (Cnidaria:Hydrozoa) and developed molecular markers to detect chimerism in both histocompatible and histoincompatible settings. Histocompatible chimeras exhibited markedly higher growth rates and survivorship than histoincompatible pairings. Histoincompatible chimeras were unstable, with chimerism being undetectable by 4 wk of age. In contrast, colonies generated from histocompatible pairings remained chimeric at markedly higher frequencies and longer durations. Histoincompatible chimeras that lost detectable chimerism retained the fusibility/rejection characteristics of the remaining component of the chimera but not that of the lost component. Chimerism across histocompatibility barriers in an invertebrate model organism was unstable and did not induce tolerance.

Endothelial cells modify the costimulatory capacity of transmigrating leukocytes and promote CD28-mediated CD4(+) T cell alloactivation

Activated vascular endothelial cells (ECs) express major histocompatibility complex (MHC) class II molecules in vitro and in vivo in acute and chronic allograft rejection. However, human ECs may be limited in their ability to effectively activate CD4(+) T cells, because they do not express members of the B7 family (CD80 and CD86) of costimulatory molecules. In this study, we show that ECs promote the full activation of CD4(+) T cells via trans-costimulatory interactions. By reverse transcriptase polymerase chain reaction, Western blot, and FACS((R)) analysis, we could not detect the expression of CD80 and CD86 on activated ECs and found minimal expression on purified CD4(+) T cells. In contrast, both CD80 and CD86 were expressed in allogeneic CD4(+) T cell-EC cocultures. Expression of CD86 peaked at early times between 12 and 24 h after coculture, whereas CD80 was not expressed until 72 h. Addition of anti-CD86 but not anti-CD80 monoclonal antibodies to cocultures inhibited IL-2 production and the proliferation of CD4(+) T cells to allogeneic donor human umbilical vein ECs (HUVECs), as well as to skin and lung microvascular ECs. Furthermore, we found that interferon gamma-activated ECs but not untreated ECs induced mRNA and cell surface expression of CD80 and CD86 on CD4(+) T cells, and these T cells were functional to provide a trans-costimulatory signal to autologous CD4(+) T cells. Blockade of MHC class II and lymphocyte function-associated antigen 3 but not other EC cell surface molecules on IFN-gamma-activated ECs inhibited the induction of CD86 on CD4(+) T cells. Transmigration of purified populations of monocytes across EC monolayers similarly resulted in the induction of functional CD86, but also induced the de novo expression of the cytokines interleukin (IL)-1alpha and IL-12. In addition, EC-modified monocytes supported enhanced proliferation of allogeneic and autologous CD4(+) T cells. Taken together, these data define the ability of the endothelium to modify CD4(+) T cells and monocytes for trans-costimulatory events. This unique function of the endothelium in alloimmune T cell activation has functional consequences for the direct and the indirect pathways of allorecognition.