Inhibition of cytotoxic alloreactivity by human allogeneic mononuclear cells: evidence for veto function of CD2⁺ cells

Abstract

In animal models of organ transplantation, infusion of donor-derived leucocytes or bone marrow cells can support tolerance induction. To date, little is known about the suppressive effects of human allogeneic mononuclear cells on alloreactivity in the human system. To study this, mixed leucocyte cultures (MLC) were incubated in the presence and absence of viable allogeneic mononuclear cells (MNC) (modulator cells) of stimulator/donor origin, and the cytotoxic and proliferative potential of the resulting effector cells was determined. The experiments showed that: viable allogeneic MNC from bone marrow and from lymph nodes and peripheral blood (PBMC) were able to suppress allospecific cytotoxicity by an average of 60%; that allospecific as well as non-specific inhibitory effects could be observed with unseparated PBMC; that CD2+ PMNC showed predominantly allospecific inhibition of cytotoxicity with little effect on proliferation whereas CD2- PBMC showed non-specific inhibitory effects (both for cytotoxicity and proliferation), which could be eliminated by indomethacin; that addition of interleukin-2 (IL-2) up to 50 U/ml to the MLC could not reverse the inhibitory effect; and that selective removal of CD8+ cells from the CD2+ modulator population diminished the specific inhibitory effect only partially. These findings demonstrate that viable human MNC from different compartments can have a marked suppressive effect on alloreactivity in vitro. For peripheral blood mononuclear cells (PBMC) the data suggest that various mechanisms can contribute to allosuppression, including specific suppressive veto effects by CD2+ cells. Such inhibitory effects might be applicable in vivo for down-regulating allospecific cytotoxicity and to facilitate the acceptance of allografts.

Fas Ligand Enhances Hematopoietic Cell Engraftment Through Abrogation of Alloimmune Responses and Nonimmunogenic Interactions

“…The early immunomodulation achieved by expression of FasL protein on the surface of HSPC in concert with the insensitivity of these cells to Fas‐mediated apoptosis endows the hematopoietic progenitors themselves with a potent regulatory activity against the host alloreactive responses, as previously suggested [31, , –34]. The early timing of this veto activity preceded the eventual early evolution of donor‐derived myeloid cells with immune capacity [38, 39]. Thus, it is the hematopoietic precursors themselves that are involved in presentation of the donor antigens, primarily the major histocompatibility antigens in the fully allogeneic transplants [34].…”

“…The upregulation of Fas and FasL expression on bone marrow‐homed donor HSPC indicated that this molecular interaction is relevant to the very early stages of engraftment. Therefore, we used a FasL protein that has a very short lifetime in vivo [13, 37] and is diluted during the differentiation myeloid progenitors that may generate immunogenic activities [38, 39]. The protein aggregates spontaneously to deliver potent apoptotic signals through trimerization of the Fas receptor and also may be adhered to the surface of cells via biotinylation.…”

The functional relevance of passenger leukocytes and microchimerism for heart allograft acceptance in the rat

“…et al, unpublished data), perhaps donor T cells are responsible for the functional effect seen. Potential mechanisms include induction of anergy in recipient T cells by non-professional presentation of alloantigens by transferred donor T cells 24,25 ; limited graftversus-host effects, causing a suppression of the recipient's immune system 26 ; or 'veto' effects, which have been demonstrated in rodent 27 as well as monkey 28 and human systems 29 . However, these immunological effects of early postoperative microchimerism are different from those of stable mixed allogeneic (macro)chimerism, which is associated with stable donor-specific transplantation tolerance due to thymic, deletional mechanisms 30,31 .…”

Interactions of allogeneic human mononuclear cells in the two-way mixed leucocyte culture (MLC): influence of cell numbers, subpopulations and cyclosporin

“…in the presence of either cyclosporin or tacrolimus. Thus, several of the parameters present in the setting of clinical organ transplantation favour prolonged survival of the passenger cells and may thereby enable functional effects of passenger lymphocytes on the recipient's immune system [20–23]. To what extent two‐way interactions between allogeneic immunocompetent cells early after organ transplantation are also important for the development of long‐term microchimerism, a phenomenon frequently observed after solid organ transplantation, [24–26] remains speculative.…”