Modulatory effects of human donor bone marrow cells on allogeneic cellular immune responses

Differentiation of regulatory myeloid and T-cells from adult human hematopoietic stem cells after allogeneic stimulation

Introduction

Donor hematopoietic stem cell (DHSC) infusions are increasingly being studied in transplant patients for tolerance induction.

Methods

To analyze the fate of infused DHSCs in patients, we developed an in vitro culture system utilizing CD34+DHSCs stimulated with irradiated allogeneic cells in cytokine supplemented medium long-term.

Results

Flow cytometric analyses revealed loss of the CD34 marker and an increase in CD33+ myeloid and CD3+ T-cell proportion by 10.4% and 72.7%, respectively, after 21 days in culture. T-cells primarily expressed TcR-αβ and were of both CD4+ and CD8+ subsets. Approximately 80% of CD3+ T cells lacked expression of the co-stimulatory receptor CD28. The CD4+ compartment was predominated by CD4+CD25+CD127-FOXP3+ Tregs (>50% CD4+CD127- compartment) with <1% of all leukocytes exhibiting a CD4+CD127+ phenotype. Molecular analyses for T-cell receptor excision circles showed recent and increased numbers of TcR rearrangements in generated T cells over time suggesting de novo differentiation from DHSCs. CD33+ myeloid cells mostly expressed HLA-DR, but lacked expression of co-stimulatory receptors CD80 and CD83. When studied as modulators in primary mixed lymphocyte reactions where the cells used to stimulate the DHSC were used as responders, the DHSC-lines and their purified CD8+, CD4+, CD33+ and linage negative subsets inhibited the responses in a dose-dependent and non-specific fashion. The CD8+ cell-mediated inhibition was due to direct lysis of responder cells.

Discussion

Extrapolation of these results into the clinical situation would suggest that DHSC infusions into transplant recipients may generate multiple subsets of donor "chimeric" cells and promote recipient Treg development that could regulate the anti-donor immune response in the periphery. These studies have also indicated that T cell maturation can occur in vitro in response to allogeneic stimulation without the pre-requisite of a thymic-like environment or NOTCH signaling stimulatory cell line.

Cellular and functional biomarkers of clinical transplant tolerance

Development of tolerance protocols requires assays or biomarkers that distinguish tolerant recipients from non-tolerant ones to be established. In addition, a thorough understanding of the plausible mechanisms associated with clinical transplant tolerance is necessary to take the field forward. Unlike the majority of molecular signature analyses utilized by others, the emphasis of this article is on the cellular and functional biomarkers of induced transplant tolerance. Immunity to an organ transplant is very complex, comprised of two broad categories - innate and acquired or adaptive immune responses. Innate immunity can be avoided by eliminating or preventing ischemic injuries to the donor organ and tolerance at the level of adaptive immunity can be induced by infusions of a number of cellular products. Since adaptive immune response consists of inflammatory hypersensitivity, cellular (cytotoxic and helper) and humoral aspects, all these need to be measured, and the recipients who demonstrate donor-specific unresponsiveness in all can be considered tolerant or candidates for immunosuppression minimization and/or withdrawal. The mechanisms by which these agents bring about transplant tolerance include regulation, anergy, exhaustion, senescence and deletion of the recipient immune cells. Another proven mechanism of tolerance is full or mixed donor chimerism. However, it should be cautioned that non-deletional tolerance can be reversed.

Generation and Characterization of Alloantigen-Specific Regulatory T Cells For Clinical Transplant Tolerance

Donor-specific CD4⁺CD127⁻CD25⁺FOXP3⁺ regulatory T cells (AgTregs) have the potential to induce clinical transplant tolerance; however, their expansion ex vivo remains challenging. We optimized a novel expansion protocol to stimulate donor-specific Tregs using soluble 4-trimer CD40 ligand (CD40L)-activated donor B cells that expressed mature antigen-presenting cell markers. This avoided the use of CD40L-expressing stimulator cells that might otherwise result in potential cellular contamination. Purified allogeneic “recipient” CD4⁺CD25⁺ Tregs were stimulated on days 0 and 7 with expanded “donor” B cells in the presence of IL-2, TGFβ and sirolimus (SRL). Tregs were further amplified by polyclonal stimulation with anti-CD3/CD28 beads on day 14 without SRL, and harvested on day 21, with extrapolated fold expansion into the thousands. The expanded AgTregs maintained expression of classical Treg markers including demethylation of the Treg-specific demethylated region (CNS2) and also displayed constricted TcR repertoire. We observed AgTregs more potently inhibited MLR than polyclonally expanded Tregs and generated new Tregs in autologous responder cells (a measure of infectious tolerance). Thus, an optimized and more clinically applicable protocol for the expansion of donor-specific Tregs has been developed.

Immunomodulation by Mesenchymal Stromal Cells and Their Clinical Applications

Mesenchymal stromal cells (MSCs) are multipotent progenitor cells that can be isolated and expanded from various sources. MSCs modulate the function of immune cells, including T and B lymphocytes, dendritic cells, and natural killer cells. An understanding of the interaction between MSCs and the inflammatory microenvironment will provide critical information in revealing the precise in vivo mechanisms involved in MSCs-mediated therapeutic effects, and for designing more practical protocols for the clinical use of these cells. In this review we describe the current knowledge of the unique biological properties of MSCs, the immunosuppressive effects on immune-competent cells and the paracrine role of soluble factors. A summary of the participation of MSCs in preclinical and clinical studies in treating autoimmune diseases and other diseases is described. We also discuss the current challenges of their use and their potential roles in cell therapies.

Systemic immunoregulatory and proteogenomic effects of tacrolimus to sirolimus conversion in liver transplant recipients

Immunosuppression (IS) withdrawal from calcineurin inhibitors is only possible in ≈ 20% of liver transplant recipients. However, mammalian target of rapamycin inhibitors (e.g., sirolimus; SRL) appear to be more immunoregulatory and might promote a tolerant state for withdrawal. Our aim was to determine whether systemic (i.e., blood, marrow, and allograft) signatures of immunoregulation are promoted by conversion from tacrolimus (TAC) to SRL. We therefore performed the following serial assays before and after SRL conversion in liver transplant recipients to test for enhanced markers of immunoregulation: (1) flow-cytometry immunophenotyping of peripheral blood mononuclear cells (PBMCs) and bone marrow aspirates for regulatory T cells (Tregs) (e.g., CD4(+) CD25(+++) FOXP3(+) ) and regulatory dendritic cells (DCregs) (immunoglobulin-like transcript 3(+) /4(+) ); (2) liver biopsy immunohistochemical staining (e.g., FOXP3:CD3 and CD4:CD8 ratios) and immunophenotyping of biopsy-derived Tregs after growth in culture; (3) effects of pre- versus postconversion sera on Treg generation in mixed lymphocyte reactions; (4) peripheral blood nonspecific CD4 responses; and (5) peripheral blood gene transcripts and proteomic profiles. We successfully converted 20 nonimmune, nonviremic recipients (age, 57.2 ± 8.0; 3.5 ± 2.1 years post-liver transplantation) from TAC to SRL for renal dysfunction. Our results demonstrated significant increases in Tregs in PBMCs and marrow and DCregs in PBMCs (P < 0.01) after conversion. In biopsy staining, FOXP3:CD3 and CD4:CD8 ratios were significantly higher after conversion and a number of biopsy cultures developed new or higher FOXP3(+) cell growth. Nonspecific CD4 responses did not change. Both pre- and postconversion sera inhibited mixed lymphocyte reactions, although only TAC sera suppressed Treg generation. Finally, 289 novel genes and 22 proteins, several important in immunoregulatory pathways, were expressed after conversion.

CONCLUSIONS

TAC to SRL conversion increases systemic Tregs, DCregs, and immunoregulatory proteogenomic signatures in liver transplant recipients and may therefore facilitate IS minimization or withdrawal.

Microchimerism in promoting graft acceptance in clinical transplantation

PURPOSE OF REVIEW

Infusions of bone marrow-derived cells together with 'space making' continue to be tested in clinical organ transplant tolerance protocols. These trials are based on the hypothesis that this might produce initial multilineage chimerism. There is some evidence that this in turn induces regulatory cells that control alloimmunity. Although a wealth of knowledge is available from animal models, this review deals with what we know or can speculate about donor bone marrow cells and chimerism in human organ transplantation.

RECENT FINDINGS

Calcineurin inhibitors are employed in most of these protocols to blunt the initial immune response. One protocol also has a stepwise regulatory cell generating treatment with sirolimus before total withdrawal. A number of donor chimeric lineages including stem cells, dendritic cells, myeloid precursors, and various lymphoid subpopulations have been described. Currently, it is recognized that the nature of cells that make up the chimerism could influence graft rejection versus acceptance. Tolerogenic donor chimeric cells may also generate regulatory subsets, thus controlling alloimmunity on two fronts.

SUMMARY

It might be speculated that prolonged and sustained regulation or possible anergy induced by chimerism may eventually lead to clonal deletion, thereby bringing about classical immunologic tolerance.

Favoring the risk-benefit balance for upper extremity transplantation--the Pittsburgh Protocol

Upper extremity transplantation is an innovative reconstructive strategy with potential of immediate clinical application and the most near-term pay-off for select amputees, allowing reintegration into employment and society. Routine applicability and widespread impact of such strategies for the upper extremity amputees with devastating limb loss could be enabled by implementation of cellular therapies that integrate and unify the concepts of transplant tolerance induction with those of reconstructive transplantation. Such therapies offer the promise of minimizing the risks, maximizing the benefits and optimizing outcomes of these innovative procedures.

Immune "tolerance profiles" in donor bone marrow infused kidney transplant patients using multiple ex vivo functional assays

Ex vivo identification of donor-specific unresponsiveness in organ transplant recipients is important for immunosuppression (IS) minimization. We tested three groups of stable living, related-donor kidney transplant patients up to 11 years postoperatively, i.e., 20 haploidenticals with donor bone marrow cell (DBMC) infusions, eight noninfused haploidentical controls (haplo controls), and 11 HLA-identical controls (HLA-id), using multiple ex vivo immune assays. We observed that no patients developed donor-specific antibodies. The majority showed donor-specific CTL unresponsiveness from year 1 onward. Thirteen of 20 DBMC recipients became specifically donor MLR nonreactive. Depletion of donor cells in DBMC recipients still MLR reactive increased donor-specific reactivity by 75% +/- 36% (p = 0.04). Adding them back in low concentration caused antigen specific inhibition. The frequencies of ELISPOT granzyme-B and interferon-gamma-producing cells somewhat paralleled the CTL and MLR responses. In the trans vivo DTH, 14 of 19 DBMC recipients demonstrated donor-specific unresponsiveness and 16 of 19 showed "linked suppression," vs none of eight and one of eight haplo controls and vs six of 10 and one of 10 HLA-ids, respectively. Most importantly, when all six assays were performed simultaneously, 10 of 18 DBMC, five of 10 HLA-ids, and no haplo controls were specifically donor unresponsive long term. We propose that a cluster analysis combining these assays will reveal tolerant recipients in whom IS minimization may safely be tested. This appears to have occurred in many DBMC-infused recipients.

Induction of Auto-reactive Regulatory T Cells by Stimulation with Immature Autologous Dendritic Cells

We have shown in ex vivo studies in donor bone marrow-infused kidney transplant recipients, that chimeric cells of either donor or recipient origin taken from the recipient's bone marrow down-regulated the recipient's cellular immune responses. In the present study, we have now induced regulatory T cells from peripheral blood mononuclear cells (PBMC) of renal transplant recipients or laboratory volunteers by multi-stimulation with autologous immature dendritic cell (iDC) enriched populations derived from either bone marrow cells (BMC) of the (immunosuppressed) kidney transplant recipients or PBMC of the laboratory volunteers (i.e., ibDC and ipDC, respectively). These regulatory T cells, induced by ibDC and ipDC, were autoreactive and designated as TAb and TAp with similar phenotypes and functional profiles. They were largely CD4 + CD25high, CD45RA low and CD45RO high, and uniformly expressed intracellular CTLA-4, and message of IL-4, IL-10, Foxp3, and differentially expressed TGFbeta. Their proliferative responses to autologous mature dendritic stimulating cells (mDC) were approximately two-fold stronger than to allogeneic mDC, and to allogeneic mDC were significantly lower than those of (control) autologous TPBL, suggesting an anergic state. TAb and TAp were not cytotoxic to autologous cells expressing Epstein-Barr virus (EBV) antigens, but were able to inhibit (regulate) the effector phase of this TPBL response to both autologous and allogeneic EBV lymphoblasts. This regulation appeared to require cell-to-cell contact.

Evaluation of the tolerogenic effects of donor bone marrow cells using a severe combined immunodeficient mouse-human islet transplant model

The immunoregulatory role of human donor bone marrow cells (DBMC) has been studied extensively in our laboratory using in vitro and ex vivo assays. However, new experimental systems that can overcome the limitations of tissue culture assays but with more clinical relevance than purely animal experimentation, needed to be generated. Therefore we have developed a new human peripheral blood lymphocyte (PBL) severe combined immunodeficient (SCID) mouse islet transplantation model without the occurrence of graft-versus-host disease (GvHD) and have used it to evaluate the tolerogenic effects of DBMC. Nonobese diabetogenic (NOD)-SCID mice were transplanted with human deceased donor islets and were reconstituted with human PBL (allogeneic to islets; denoted as recipient) with or without DBMC from the islet donor. It was observed that the most cellularly economical dose was 3000 islets per animal and that injection into the portal vein was better than implantation under the kidney capsule. Even though maximal lymphoid reconstitution was observed with 40-million fresh and anti-CD3 activated recipient PBL (conventional method), the mice developed severe graft GvHD. However, with the new method of reconstitution where animals were injected with 20-million anti-CD3-activated plus 40-million anti-donor-activated recipient PBL, no discernible GvHD was observed. More importantly, this latter method was associated with islet transplant rejection, which in turn could be abrogated by co-injection of the animals with DBMC. These in vivo results confirmed our previous in vitro observations that human DBMC have regulatory activity.

Mesenchymal stem cells in immunoregulation

Mesenchymal stem cells are present within the bone marrow cavity and serve as a reservoir for the continuous renewal of various mesenchymal tissues. Recent studies suggest that mesenchymal stem cells modulate immune reactions in vitro and escape from immune surveillance in vivo. We provide herein a discussion of issues including the current research progress on the in vitro interactions of mesenchymal stem cells with multiple subsets of immune cells (dendritic cells, T cells, B cells and NK cells), in vivo transplantation outcomes, the possible underlying mechanisms, future research directions as well as potential clinical implications.

Campath-1H does not alter bone marrow cell regulatory function

We have previously reported in laboratory volunteers (in vitro) and renal transplant recipients (ex vivo) that bone marrow cells (BMC) are potent downregulators of the immune response. Also, the use of alemtuzumab (Campath-1H, C1H) for immunodepletion is associated with the most potent lasting effects yet seen on T-cell immunity in renal transplantation. We questioned whether the administration of C1H to kidney allograft recipients of donor bone marrow cell (DBMC) infusions would lead to stronger or weaker immunoregulatory effects. Human BMC depleted of T cells (nT-BMC) were either untreated or treated with C1H and rabbit complement and compared for their ability to downregulate autologous or allogeneic T-cell responses and to generate T regulatory (T reg) cells. The proliferative responses to anti-CD3 monoclonal antibody of T cells derived from cocultures with C1H-treated or untreated autologous nT-BMC were equally suppressed, i.e., an equivalent alteration in CD3 complex signaling, not regained by the addition of interleukin 2. Adenosine triphosphate levels were also markedly reduced in T cells both from C1H-treated and untreated nT-BMC cocultures. The ability of C1H-treated or untreated nT-BMC to suppress autologous T-cell cytotoxic function was also equivalent, with a marked, but equivalent, capacity to induce CD4/CD25(high) T regs from CD3(+) cells, which effectively downregulated cytotoxic T cells. To mimic the clinical infusion of DBMC into (allogeneic) recipients, peripheral blood mononuclear cells were also cultured with allogeneic C1H-treated and untreated nT-BMC. T cells derived from these cultures secondarily stimulated with the same-donor mature antigen-presenting cells exhibited suppressed cytotoxicity by 85% and 54%, respectively. These in vitro studies suggest that C1H does not abrogate BMC immunoregulation and thus may allow its lympho-depleting effect to be synergistic.

Antigen presentation and immune regulatory capacity of immature and mature-enriched antigen presenting (dendritic) cells derived from human bone marrow

Several reports including those from this laboratory have demonstrated that bone marrow cells (BMC) downregulate in vitro both mixed leukocyte reaction and cytotoxic T lymphocyte reactions. We consequently hypothesized that a general property of immature cells of hematopoietic organs is their ability to suppress immune reactivity. As one of these suppressive activities, the lack of costimulatory molecules was proposed as a mechanism by which immature antigen presenting cells of the bone marrow might be involved. In the present report, we used two culture environments, each of which would regulate a different maturation pattern of human bone marrow-derived enriched dendritic antigen presenting cells (DC or APC) to determine the respective effects on in vitro immune regulatory function. Human BMC depleted of CD3+ cells were cultured with either: interleukin-4 (IL-4) and granulocyte macrophage-colony stimulating factor (GM-CSF), to maintain DC-enriched populations in an immature state (iAPC); or an interferon-gamma (IFNgamma), tumor necrosis factor alpha (TNF-alpha), GM-CSF, LPS, and IL-6 cocktail to promote the maturation of DC-enriched APC (mAPC). These iAPC and mAPC were, respectively, phenotypically characterized and also tested in vitro for the following: (1) both direct and indirect-antigen presentation functions; (2) immune regulatory functions on the response of autologous and allogeneic peripheral blood lymphocytes (PBL); and (3) Western blot analysis determining the levels of both major histocompatibility complex (MHC) class I related cytoplasmic transporter molecules associated with antigen processing (TAP1) and as well as proteasome activator molecules (PA28alpha). The iAPC population expressed fewer dendritic cell markers (CD83 and DCsign), and costimulator molecules (CD86 and CD40) than the mAPC, such that there was an approximate threefold increase in expression of CD83, 2.5-fold increase in DCsign, and a threefold increase in CD40 and CD86 on mAPC than on iAPC (p=0.005 for CD83; p=0.001 for DCsign; p=0.001 for CD86; and p=0.001 for CD40). In lymphoproliferative assays, indirect and direct alloantigen presentation by iAPC was weaker than by mAPC (p=0.05 and 0.04). In addition, iAPC were able to downregulate allogeneic CTL responses. Also, after pulsing with Epstein-Barr virus (EBV) protein antigens, the iAPC were less efficient in their presentation to autologous EBV-specific T-cell lines, and caused an inhibition of EBV-CTL generation. The expression of TAP1 and PA28alpha was reduced in iAPC in comparison to mAPC. These findings support the notion that a maturation state of BMC-derived APC correlates with their capacity to present antigen. The observed in vitro deficiency of this function by immature bone marrow cells may therefore contribute to the immune downregulatory capacity seen in the BMC compartment.

Allogeneic bone marrow inhibits T-cell activation and clonal expansion in vitro

BACKGROUND

Donor bone marrow infusion has long been used to enhance graft survival or induce tolerance in T cell depleted solid organ allograft recipients. However, the mechanisms through which bone marrow cells affect tolerance remain obscure. We studied the affect of allogeneic bone marrow cells on the activation of allospecific T cells in vitro.

METHODS

Carboxyfluorescein-diacetate succinimidyl ester-labeled CBA/Ca strain CD8+ splenocytes, bearing T-cell receptor alpha and beta transgenes from the BM3.3 T-cell clone specific for the major histocompatibility complex class I antigen Kb, were placed in culture with irradiated C57BL/6J stimulator cells in the presence of increasing numbers of C57BL/6J or Balb/cJ bone marrow cells for 1 to 3 days. Responder cells were individually analyzed for proliferative history, expression of activation-associated antigens, and intracellular cytokine production.

RESULTS

Allogeneic bone marrow cells exert a dose-dependent inhibitory effect on proliferation of allospecific CD8+ T cells in mixed lymphocyte culture. However, the inhibited T-cell subpopulations show physiologic changes associate with the early stages of T-cell activation, including expression of CD69 and early decrease of surface T-cell expression. Unlike cells not co-cultured with bone marrow, these cells fail to reexpress the T-cell receptor (TCR) by 72 hr of culture. The observed inhibitory effect is also associated with a decrease in the proportion of CD8+ cells expressing interleukin-2 and interferon-gamma.

CONCLUSIONS

Collectively, these results suggest that peripheral allospecific T cells undergo the initial stages of activation on exposure to antigen in the presence of bone marrow cells, but the cell cycle is arrested and TCR reexpression is inhibited. We speculate that bone marrow cells effect this inhibition through a receptor-ligand interaction that modulates the transmembrane signal pathway for the TCR.

Immune responses and their regulation by donor bone marrow cells in clinical organ transplantation

Infusions of donor bone marrow derived cells (DBMC) continue to be tested in clinical protocols intended to induce specific immunologic tolerance of solid organ transplants based on the observations that donor-specific tolerance is induced this way in animal models. We studied the immunological effects of human DBMC infusions in renal transplantation using modifications in lymphoproliferation (MLR) and cytotoxicity (CML) assays. The salient observations and tentative conclusions are summarized in this review. Among many types of organs transplanted using DBMC at this center, it was found that the cadaver renal recipients (CAD) had significantly decreased chronic rejection and higher graft survival when compared to equivalent non-infused controls. DBMC infusion was also associated with a marginal and non-specific immune depression. It was also observed that the number of chimeric donor cells gradually increased in the iliac crest bone marrow compartment with a concomitant decrease in the peripheral blood and that the increase was more rapid in living-related donor (LRD)-kidney/DBMC recipients in spite of a lower number of DBMC infused (<25%) than in the CAD-kidney/DBMC group. In the LRD recipients with residual anti-donor responses, purified chimeric cells of either donor or recipient inhibited recipient immune responses to the donor significantly more strongly than the freshly obtained bone marrow from the specific donor or volunteer suggesting an active regulatory role for chimeric cells. A number of (non-chimeric) subpopulations of bone marrow cells including CD34(+) stem cells and the CD34(-) early progeny like CD38(+), CD2(+), CD5(+) and CD1(+) lymphoid cells as well as CD33(+) (but CD15(-)) myeloid cells down-regulated the MLR and CML responses of allogeneic PBMC stimulated with (autologous) donor spleen cells. These regulatory effects appeared to be refractory to the action of commonly used immunosuppressive drugs and occurred during the early phase of the immune response through cell-cell interactions. Most of these DBMC sub-populations had stimulatory capabilities, albeit markedly lower than donor spleen cells, but only through the indirect antigen presentation pathway. When co-cultured with allogeneic stimulators, purified CD34(+) cells were found to give rise both to CD3(-) TCRalphabeta(+), as well as CD3(+) TCRalphabeta(+) cells and, thereby, responded in MLR to allogeneic stimulation (but did not generate cytotoxic effector cells). Also, a number of DBMC subpopulations inhibited the CML and to a lesser extent the MLR, of autologous post-thymic responding T cells stimulated with allogeneic irradiated cells, mediated through soluble factors. Finally, non-chimeric DBMC also inhibited the proliferative and cytotoxic responses of autologous T cells to EBV antigens, inducing T suppressor cells, which in turn could inhibit autologous anti-EBV CTL generation and B cell anti-CMV antibody production. These studies all suggested a strong inhibitory property of a number of DBMC sub-populations in vitro and in vivo with the notion that they promote unresponsiveness.

A novel micro-cell-mediated lympholytic assay for the evaluation of regulatory cells in human alloreactive CTL responses

Since cell-mediated lympholysis (CML), the most commonly used in vitro experimental cytotoxic method for the evaluation of regulatory cells, requires large numbers of cells that are often the limiting factor, we have developed a new micro-cell-mediated lympholytic (m-CML) assay. Various numbers of responding cells were stimulated with equivalent numbers of allogeneic irradiated stimulator cells in the presence of (fivefold) serial dilutions of regulatory cells. On the 8th day, 4-h 51Cr-release assays were performed by adding 5000 labeled target cells from the corresponding stimulators to the cultures. Even though results that were comparable to the macro- (bulk) CML and MLR modulation experiments were obtained with all the cell combinations tested in the m-CML, the combinations with 50,000 responder cells and stimulating cells and dilutions of 25,000 to 40 modulator (regulatory) cells were found to be the most reproducible for assaying regulatory cell potency in vitro. Similarly, expression of the results as percentage inhibition using percent specific lysis values was the simplest method of calculation. This assay was standardized for the evaluation of the inhibitory activity of a variety of regulatory cells, including long-term cultures of cadaver-donor vertebral body bone marrow cells (vDBMC-L), in vitro generated CD8 positive and CD28 negative suppressor T cells and donor chimeric cells isolated from renal transplant recipients who had been perioperatively infused with donor bone marrow cells (DBMC). The results indicate that the m-CML assay is a sensitive and reliable micromethod with at least 10-fold fewer responders, stimulator and modulator cell numbers needed than macro-CML assays for the evaluation of regulatory cells obtained from a variety of immune systems in vitro.

Human donor bone marrow cells induce in vitro "suppressor T cells" that functionally suppress autologous B cells

We have reported a beneficial effect of donor vertebral body bone marrow cells (DBMC) infusions in cadaver renal allograft recipients in a 6-year follow-up, but with a transient increase in early (6 month) postoperative CMV infections and concomitant suppressed immunoglobulins (Ig) production. We also found that although there was no difference between the DBMC-infused and non-infused (control) groups in the development of donor-specific antibody, we now describe an additional difference seen in the percent reactive antibody (PRA) reactivity against a panel of HLA antigens that developed postoperatively. We hypothesize that (allogeneic) antigen presenting cells in the DBMC, systemically infused, caused the generation of recipient T suppressor (T4-suppressor) cells, thereby "inducing" a negative influence on B cell Ig production. We tested this notion in vitro by incubating PBL from CMV IgG positive laboratory volunteers with either (allogeneic) T-cell depleted DBMC or donor spleen cells (DSPC) from (the same) cadaver donors. After 7 days, the (responding) T cells were collected using magnetic beads and placed in culture with purified B cells freshly obtained from the same (autologous) CMV positive volunteer. To these cultures were added either media or 40 ng of CMV antigen. After 3, 5, 7, and 9 days, the expression of surface anti-CMV Ig was measured by flow cytometry using a panel of fluorescent markers double-labeled for activated B cells (CD20, CD19, and HLA DRw) and CMV-FITC. We also determined the phenotype of the cultured T cells using anti-CD3, CD4, and CD62L specific monoclonal antibodies. B cells that had been in contact with autologous T cells derived from DBMC cultures (TBM) were less likely to express anti-CMV surface Ig than those cultured with DSPC (TSP). The flow cytometry analysis revealed an increase in the number of T4 suppressor cells (CD3+, CD4+, CD62L+) in the TBM group, whereas the T4 helper phenotype (CD3+, CD4+, CD62L-) predominated in the TSP group. These in vitro findings support the notion that (allogeneic) DBMC infusions can induce a T4 suppressor (regulatory) influence and thereby indirectly affect B-cell function.

Regulation of alloimmune responses (GvH reactions) in vitro by autologous donor bone marrow cell preparation used in clinical organ transplantation

BACKGROUND

Cadaver donor bone marrow cells (DBMC) are capable of a low-grade response to allogeneic stimulation in vitro, indicating their potential ability to cause graft versus host disease (GvHD). However, at this center, we have observed a lack of GvHD in kidney transplant recipients who received DBMC perioperatively. Therefore, we questioned whether an intrinsic immunoregulatory function of (subpopulations of) DBMC might play a role in this observation.

METHODS

In in vitro assays, DBMC was added to autologous splenic responder cells taking part in allogeneic mixed lymphocyte reaction (MLR) and cell-mediated lympholysis (CML) reactions.

RESULTS

When compared with autologous donor irradiated spleen cells as control modulators, DBMC significantly inhibited the CML, but to a much lesser extent than MLR, of autologous responding cells stimulated with allogeneic irradiated cells in a dose dependent manner. The down-regulation of CML responses was observed even in the presence of pharmacological concentrations FK506, mycophenolic acid, or cyclosporine-A. The inhibition could not be overcome by the addition of exogenous helper factors. Moreover, in contrast with findings previously reported from this laboratory of the in vitro inhibitory effect of DBMC on allogeneic responding cells in MLR and CML reactions to stimulating cells of the DBMC donor (allogeneic host versus graft inhibition), the following unique observations were made using DBMC inhibiting autologous reactions (GvH): (1) optimal restimulation of autologous responder cells in secondary cultures could not abrogate this inhibition; even activated responder cells could be inhibited by autologous DBMC, and (2) soluble factors were at least partially operative in this autologous (GvH) inhibition as indicated by experiments in transwells and by the CML inhibition caused by 50% supernatants from DBMC cultures.

CONCLUSION

These in vitro results indicate that DBMC treatment brings about a marked down-regulation of autologous immunocompetent cells in cytotoxicity reactions, partially at least through secreted soluble factor(s), (and in presence of immunosuppressive drugs in vivo) thereby preventing overt clinical GvHD.

Six-year clinical effect of donor bone marrow infusions in renal transplant patients

BACKGROUND

To date, several single- and multicenter clinical trials have attempted to induce specific immunological unresponsiveness using donor bone marrow cell infusions to augment solid organ transplantation, but the outcomes have not been definitive.

METHODS

Between September 1994 and May 1998, 63 cadaver (CAD) renal transplant recipients of either one or two postoperative donor bone marrow cell (DBMC) infusions were prospectively compared with 219 non-infused controls given equivalent immunosuppression. There was at least a 1 HLA DR antigen match present between donors and recipients. The immunosuppressive regimen included a 10-day course of OKT3 induction, and tacrolimus, mycophenolate mofetil, and methylprednisolone maintenance. A total 7.01x10(8)+/-1.9x10(8) (SD) DBMC/kg was infused into the CAD recipients on either days 4 and 11 (n=42) or one half of that dose on day 4 (n=21) postoperatively. Clinical follow-up has ranged from 2.9 to 6.3 years (mean, 4.7 years). Studies were also performed of humoral immunity and quantitative cellular chimerism.

RESULTS

There is clear-cut equivalence in immunosuppressive dosaging and in the other major demographic variables in both groups. However, only 2/63 DBMC recipients had biopsy-proven chronic rejection, whereas 41/219 showed chronic rejection in the controls (P = <0.01). In both groups, mortality was not associated with rejection. The actuarial graft survival at 6.3 years in the CAD DBMC group was 84.3% compared with 72.2% in the control group (not statistically significant). However, if death with a functioning graft was excluded, graft survival was 94.1% in the DBMC group and 79.8% in the controls (P=0.039). Forty patients in the control group continue to have deteriorating renal function (increasing serum creatinine concentrations to 2 mg/dl and higher), compared with 2 patients in the DBMC group (P=0.04). In the DBMC group, chimerism in iliac crest marrow aspirates has increased 3-fold in yearly sequential measurements between 1 and 4 years postoperatively averaging 1.3+/-0.36% (SE) most recently. This has not occurred in the controls.

CONCLUSIONS

There now appears to be more solid long-term evidence, in kidney transplant recipients prospectively receiving DBMC infusions, of an improvement in long-term graft survival, and of the degree of chimerism positively correlating with the absence of graft loss.

Donor bone marrow-derived chimeric cells present in renal transplant recipients infused with donor marrow. I. Potent regulators of recipient antidonor immune responses

BACKGROUND

Even though a number of transplant centers have adopted donor-specific bone marrow cell (DBMC) infusions to enhance donor cell chimerism, to date there has been no direct evidence linking chimerism with tolerance induction in human organ transplant recipients.

METHODS

Cells of donor phenotype were isolated 1 year postoperatively from the peripheral blood lymphocytes and iliac crest bone marrow of 11 living-related-donor (LRD) renal transplant recipients, who had received perioperative donor bone marrow cell infusions. These recipient-derived donor (RdD) cells were characterized phenotypically by flow cytometric analysis and functionally as modulators in mixed lymphocyte reaction (MLR) and cell-mediated lympholysis (CML) assays.

RESULTS

The yield of RdD cells ranged from 0.1 to O.9% of the starting material with the majority being TcRalphabeta, CD3 positive T cells, a substantial percentage of which coexpressed CD28. At 1 year posttransplant almost 50% of the LRD-kidney/DBMC recipients tested so far exhibited donor-specific unresponsiveness in MLR (7/17) and CML (6/13) reactions and this trend was further enhanced at 23 years. In the recipients with residual positive antidonor immune responses, the RdD cells inhibited recipient antidonor MLR and CML responses significantly more strongly than freshly isolated and similarly treated iliac crest bone marrow cells from the donor. RdD cells also inhibited the MLR of the recipient to third party allogeneic stimulator cells; however, this nonspecific effect was significantly weaker than specific inhibition. We also established long-term bone marrow cultures stimulated every 2 weeks with irradiated alogeneic feeder cells, that had similar functional properties thus possibly providing us with an in vitro correlate the RdD cells.

CONCLUSIONS

These results clearly support the notion that the infused donor cells play a positive role in the induction and/or maintenance of transplant tolerance.

Involvement of multiple subpopulations of human bone marrow cells in the regulation of allogeneic cellular immune responses

BACKGROUND

The identity of the cells in the human bone marrow that function as effective regulators of in vitro and possibly in vivo cellular immune responses is not well established.

METHODS

Cell subpopulations were isolated from cadaver donor vertebral-body bone marrow cells (DBMC) by using immuno-magnetic microbeads and were tested as inhibitors (modulators) in cell-mediated lympholysis (CML) and mixed lymphocyte reaction (MLR) responses of normal peripheral blood lymphocytes stimulated with irradiated cadaver donor spleen cells.

RESULTS

Compared with spleen cells as controls, un-irradiated T-cell depleted DBMC inhibited both the MLR and CML responses of allogeneic responder cells in a dose dependent manner (as in our previous reports). The inhibition was also mediated by a number of purified subpopulations including pluripotent CD34+ stem cells, and their CD34 negative early progeny of both lymphoid and myeloid lineages. These included DBMC enriched for non-T-cell lymphoid precursors (NT-LP/DBMC; i.e., DBMC depleted of CD3, CD15, and glycophorin-A positive cells) and DBMC positively selected for CD38+, CD2+, CD5+, and CD1+ lymphoid cells (all were depleted of CD3+ cells) as well as CD33+ (but CD15 negative) myeloid precursors. However, positively selected CD19+ B-cells and CD15+ myeloid cells did not inhibit the MLR and CML responses. The NT-LP/DBMC that had been repeatedly stimulated with irradiated allogeneic peripheral blood lymphocytes caused the strongest inhibition of the MLR and CML responses of the same allogeneic cells with 200 times fewer modulator cells needed than uncultured DBMC (P<0.001). Flow cytometric analysis revealed that majority of cells in these cell lines had become CD3+ TcR-alphabeta+ CD4+ and CD28+ cells.

CONCLUSION

A variety of less differentiated cells of various lineages residing in the human bone marrow are immunoregulatory in vitro. Among them, there is at least one subset that can undergo differentiation in vitro into regulatory T cells that can be maintained in long-term cultures.

Bone marrow cells promote TH2 polarization and inhibit virus-specific CTL generation

This laboratory recently reported that human bone marrow cells (BMC) inhibit the generation of virus-specific CTL in culture. The culture supernatants contained increased levels of prostaglandin E(2) (PGE(2)) (shown to favor TH2 cell development) and also inhibited EBV-CTL effector cell development. In this study, we obtained PBL from Epstein-Barr virus (EBV) IgG antibody positive kidney transplant recipients (R) and their living-related donors (LRD) one year after renal transplantation. EBV-specific CTL were then generated in vitro by stimulating PBL with autologous EBV-transformed B cells (EBV-B) in the presence or absence of autologous BMC. The addition of BMC to the EBV-CTL generation cultures increased the intracellular expression in CD3+ cells of IL-4,-5,-6,-10, and -13. These CD3+ cells also expressed increased levels of the TH2 associated receptor CCR3. Inhibition was even observed by preparing EBV-CTL generating cultures in trans-wells that separated the autologous BMC from the PBL + EBV-B. It was then observed that CD3+ cells obtained after 7 days of culture in the presence of autologous BMC could be used as inhibitors of EBV-CTL generation. Protein Kinase A (PKA), a cAMP kinase that is involved in the upregulation of TH2 cytokine activity, was increased in EBV-CTL cultures by the presence of BMC. Additionally, IL-4-mediated signal transduction and activation of transcription (STAT-6) phosphorylation was slightly increased. These results show that the BMC inhibition is mediated by soluble factors (cytokines) and that cell-cell contact in this autologous system is not required, so that BMC (at least partially, via cytokine production) promote TH2 polarization in culture. Moreover, TH2 cells induced by culturing with autologous BMC directly inhibit EBV-CTL generation, and TH2 associated PKA, CCR3, and STAT-6 phosphorylation are enhanced by BMC.

Posttransplant infusion of donor-specific blood induces immunological unresponsiveness in rat hepatic allografts

BACKGROUND

We previously reported that pretransplant donor-specific blood transfusion (DST) induces CD45RC-CD4+ T cells, Th2-like effector cells, and prolongs rat hepatic allograft survival. Our study investigated the effects of posttransplant DST on rat hepatic allograft survival.

METHODS

Three days after transplantation, LEW (RT1(1)) recipient rats with ACI (RT1a) livers were injected i.v. with freshly heparinized donor-specific blood. The time kinetics of CD45RC-CD4+ and CD45RC+CD4+ T cell subsets in hepatic infiltrates were examined.

RESULTS

Posttransplant DST significantly prolonged rat hepatic allograft survival. Interferon (IFN)-gamma, interleukin (IL)-12, and IL-18 mRNA levels in hepatic allografts of untreated recipients were significantly greater than in recipients treated with posttransplant DST. However, hepatic allografts of recipients treated with posttransplant DST showed significantly higher IL-4, IL-10, and transforming growth factor (TGF)-beta mRNA levels than untreated recipients. The ratio of CD45RC-CD4+ T cells to CD45RC+CD4+ T cells was significantly higher in hepatic allografts treated with posttransplant DST than in untreated animals. Immunostaining with anti-rat dendritic cell (OX-62) monoclonal antibody revealed that OX-62+ cells were distributed to the splenic red pulp of animals treated with posttransplant DST and to the splenic white pulp in untreated animals. Most OX62+ cells isolated from the spleen of recipients treated with posttransplant DST expressed donor RT1Ba class II major histocompatibility complex antigens, suggesting that OX-62+ cells were of donor origin.

CONCLUSION

Posttransplant DST was associated with persistent infiltration of CD45RC-CD4+ T cells, Th2-like effector cells, in rat hepatic allografts, causing immunologic unresponsiveness and establishment of microchimerism in the spleen.

Bone marrow cells inhibit the generation of autologous EBV-specific CTL

Recently, we reported that human bone marrow cells (BMC) inhibited the proliferative (recall) response of lymphocytes to Epstein-Barr virus (EBV) and cytomegalovirus (CMV) protein antigens [12]. To clarify further the effect of BMC on the immune response to viral antigens, we obtained PBL from EBV IgG antibody positive kidney transplant recipients (R) and their living-related donors (LRD) 1 year after renal transplantation and generated EBV-specific CTL in vitro in the presence or absence of autologous BMC. The addition of freshly aspirated autologous iliac crest BMC from either R or LRD caused a significant inhibitory effect on the generation of EBV-specific CTL from CTL precursors, in contrast to the addition of autologous PBL used as controls (62.29 +/- 10.85% inhibition using BMC from the kidney transplant recipients; 74.47 +/- 15.21% inhibition using BMC from the living-related donors). This inhibitory effect was only exerted during the CTL generation phase; but not in the effector CTL killing phase. The expression of CD94, a component of the killer inhibitory receptor (KIR) on CD3(+) cells was elevated in the cultures with BMC, in contrast to the cultures without BMC. The BMC inhibitory effect was partially abrogated by pre-incubation of the CTL effectors with anti-CD94 monoclonal antibody, in contrast with its isotype control. In addition, supernatants obtained from the CTL generating cultures with BMC contained high levels of prostaglandin E(2) (PGE(2)), and EBV-specific CTL activity was inhibited by the addition of exogenous PGE(2) in the absence of BMC. The induction of CD40L cell surface expression by anti-CD3 was also decreased on the effector T cell population when BMC were added. There was a concomitant reduction in protein kinase C (PKC) activity. These studies demonstrate that BMC exert an inhibitory effect on T cell-mediated immunity to viral antigens in humans by regulating autologous effector T cell generation and early T cell activation signaling pathways.

A clinical trial combining donor bone marrow infusion and heart transplantation: intermediate-term results

BACKGROUND

Donor chimerism (the presence of donor cells of bone marrow origin) is present for years after transplantation in recipients of solid organs. In lung recipients, chimerism is associated with a lower incidence of chronic rejection. To augment donor chimerism with the aim to enhance graft acceptance and to reduce immunosuppression, we initiated a trial combining infusion of donor bone marrow with heart transplantation. Reported herein are the intermediate-term results of this ongoing trial.

METHODS

Between September 1993 and August 1998, 28 patients received concurrent heart transplantation and infusion of donor bone marrow at 3.0 x 10(8) cells/kg (study group). Twenty-four contemporaneous heart recipients who did not receive bone marrow served as controls. All patients received an immunosuppressive regimen consisting of tacrolimus and steroids.

RESULTS

Patient survival was similar between the study and control groups (86% and 87% at 3 years, respectively). However, the proportion of patients free from grade 3A rejection was higher in the study group (64% at 6 months) than in the control group (40%; P =.03). The prevalence of coronary artery disease was similar between the two groups (freedom from disease at 3 years was 78% in study patients and 69% in controls). Similar proportions of study (18%) and control (15%) patients exhibited in vitro evidence of donor-specific hyporesponsiveness.

CONCLUSIONS

The infusion of donor bone marrow reduces the rate of acute rejection in heart recipients. Donor bone marrow may play an important role in strategies aiming to enhance the graft acceptance.

Analysis of post-transplant immune status in recipients of liver/bone marrow allografts

The aims of this study were to assess the effect of donor bone marrow infusion on the reactivity of recipient peripheral blood lymphocytes (PBL) to mitogen and to donor and third-party cells after primary liver allotransplantation and to identify any correlation between altered immunoreactivity and HLA mismatches, occurrence of rejection, and immunosuppression. The immunoreactivity of recipient PBL toward frozen donor splenocytes was evaluated in mixed lymphocyte culture (MLC) (n = 29) and cell-mediated lympholysis (CML) (n = 27) assays in time intervals ranging from 0.7 to 27 months after transplant. Overall, the mean anti-donor MLC stimulation index (SI) fell from 25.6 +/- 5.2 preoperatively to 4.8 +/- 1.7 after transplantation (p < 0.002), with 14 out of 29 (48.3%) patients developing donor-specific MLC hyporeactivity. HLA class II mismatches were significantly associated with recipient post-transplant immune profile (p < 0.05): MLC donor specific hyporesponsiveness was observed in 70%, versus 37% of patients who shared a class II antigen, versus those that did not. Of the control group, 61.1% developed donor-specific nonreactivity versus 27.2% in the donor bone marrow cells (DBMC) group (p = 0.02). Donor-specific CML hyporeactivity was observed after transplantation, independent of DBMC infusion, with mean percentage values of pre- and post-transplant donor-specific lysis of 22.4% +/- 4.1% versus 3.1% +/- 1.6%, p = 0.0004, respectively. Our results suggest that DBMC infusion favors development of nonspecific MLC hyporesponsiveness to donor and third-party alloantigen, with maintenance of reactivity to mitogen and no additional effect on T-cell cytotoxicity.

In vitro immunogenicity of cadaver donor bone marrow cells used for the induction of allograft acceptance in clinical transplantation

BACKGROUND

The cascade of immunological effects brought about by donor bone marrow cell (DBMC) infusions to induce allograft acceptance in clinical transplantation is not fully understood. Aside from acting as immune responding and regulatory cells, the infused DBMC also may sensitize the recipient to the donor antigens.

METHODS

To analyze this stimulatory activity of DBMC, in vitro mixed lymphocyte cultures (MLC) and cell-mediated lymphocytotoxicity (CML) culture systems analogous to the transplant model with DBMC infusion were used.

RESULTS

When responding peripheral blood lymphocytes (PBL) from normal volunteers were placed in culture with suspensions of Ficoll-purified, T cell-depleted, un-irradiated allogeneic DBMC (NT-DBMC), a reaction was seen in both MLC and CML. However, when compared to allogeneic spleen cells as stimulating cells, the responses to NT-DBMC were of markedly lower magnitude and were not seen when the NT-DBMC was irradiated (3000 R). When responding PBL were stimulated with either NT-DBMC that had been previously cultured with irradiated cells from the responders for 1 week (activated NT-DBMC), NT-DBMC further depleted of CD15+ and glycophorin A-positive cells (NT-LP/DBMC), or purified CD34+ and CD2+ DBMC subsets, stronger lymphoproliferative and cytotoxic responses were observed. Moreover, these responses were not abrogated by irradiation of the stimulating DBMC subpopulations. Depletion of antigen-presenting cells by positive selection of CD3+ cells from the responding PBL abrogated MLC and CML reactivity, even when purified NT-LP/DBMC, the most stimulatory cells, were used. This latter observation was in contrast to the responses seen with cultures containing allogeneic stimulating spleen cell populations. This indicated the requirement for indirect alloantigen presentation, i.e., the failure of these DBMC to stimulate by direct alloantigen presentation. NT-DBMC was able to stimulate responding PBL in secondary MLC and CML responses with an equivalent magnitude, irrespective of whether the stimulators were spleen cells or NT-DBMC. Finally, the MLC and CML responses were inhibited by tacrolimus (FK506), mycophenolic acid (MPA), and cyclosporine (CsA) in a dose dependent manner, in contrast to previously observed refractoriness of DBMC preparations to these agents if DBMC was tested as responder cells or in modulatory assays.

CONCLUSIONS

These results indicate that DBMC are able to function as effective in vitro stimulators, but only by indirect antigen presentation, and that the immune responses mediated by them can be down-regulated by their own inherent suppressive nature, an effect that can be enhanced by the presence of immunosuppressive drugs.

Simultaneous donor bone marrow and cardiac transplantation: can tolerance be induced with the development of chimerism?

Mixed bone marrow chimerism (mixed chimerism) is defined by the coexistence of two genetically different bone marrow stem cells in an individual. The chimeric immune system recognizes donor antigen as self, yet is capable of mounting a normal response to third-party antigens. In animal models, mixed chimerism confers donor-specific tolerance for solid-organ and cellular grafts: tissue from the bone marrow donor is permanently accepted by mixed chimeras in the absence of conventional immunosuppressive agents. Clinical application of mixed chimerism to induce transplantation tolerance requires novel approaches to safely and reliably achieve engraftment of donor bone marrow in transplant recipients. Recent advances offer potential solutions to these obstacles and suggest that the application of mixed chimerism to induce tolerance to transplanted organs may soon be a clinical reality.

Immunoregulatory role of CD8alpha in the veto effect

BACKGROUND

Allogeneic bone marrow cell (allo-BMC) infusion induces tolerance to incompatible renal allografts in rhesus macaques after depletion of peripheral T lymphocytes with cytolytic anti-T cell antibodies. The tolerogenic effect of allo-BMC, ascribed to a veto mechanism, associates with specific functional deletion of antidonor cytotoxic T lymphocyte precursor (CTLp), and is dependent on a CD8+ donor BMC subset. In previous studies, the CD8 molecule was implicated by loss of suppression after blocking interaction between CD8 on allo-BMC and major histocompatibility complex class Ialpha3 domain on CTLp. CD8 cross-linking on BMC induced secretion of active transforming growth factor-beta1 (TGF-beta1), suggesting a regulatory mechanism(s) operating via a CD8-mediated signaling pathway.

METHODS

CD8 on rhesus cells was cross-linked using IgG-conjugated beads, and TGF-beta1 mRNA and protein were quantified. CD8+ cells were tested for veto activity by mixed lymphocyte reaction (MLR)-induced cell-mediated lymphocytotoxicity (CML) assay. Activated rhesus T cells exposed to TGF-beta1 were examined for apoptosis by TdT-mediated end-labeling and annexin staining.

RESULTS

CD8 cross-linking induces accumulation of TGF-beta1 mRNA and protein. Both CD3- CD8+CD16+ and CD3+ CD8+CD16- subsets of allo-BMC up-regulate TGF-beta1 mRNA after CD8 cross-linking, and exhibit veto activity. The CD3-CD8+CD16+ subset expresses more TGF-beta1 mRNA and increased veto activity at low BMC/CTLp ratios. Exposure of activated T cells to TGF-beta1 induces apoptosis.

CONCLUSIONS

CD8+ allo-BMC are enriched for veto activity and activation via CD8 induces TGF-beta1 mRNA and protein accumulation. These results agree with the hypothesis that paracrine TGF-beta1 may be involved in peripheral deletion of alloreactive CTLp by CD8+ allo-BMC. We suggest that TGF-beta1 overexpression by donor lymphohematopoietic cells may enhance tolerance induction.

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

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.

Cellular immune responses of human cadaver donor bone marrow cells and their susceptibility to commonly used immunosuppressive drugs in transplantation

BACKGROUND

The cascade of immunological effects brought about by donor bone marrow cell (DBMC) infusions in human organ transplantation, especially in the context of continuous pharmacologic immunosuppression, is not fully understood. Yet, in inbred rodents and even primates, administration of specific bone marrow cells has caused a state of acquired immunologic tolerance.

METHODS

In vitro mixed lymphocyte culture (MLC) and cell-mediated lympholysis (CML) culture systems were used to compare the responding and regulatory properties of DBMC and individual bone marrow cell subsets versus spleen cells in the presence or absence of pharmacologic immunosuppression.

RESULTS

In the absence of immunosuppressive drugs, the DBMC proliferated in MLC and in response to phytohemagglutinin, but to a lower magnitude than donor spleen cells. In CML assays, DBMC failed to function as cytotoxic cells. Removal of both CD3+ and CD34+ cells together (not just singly) had to occur for complete abrogation of the proliferative response of DBMC evoked in the presence of allogeneic stimulating cells. Testing several experimental variables using flow cytometric analysis led to the conclusion that when purified DBMC CD34+ cells were placed in coculture with irradiated allogeneic peripheral blood mononuclear cells, such CD34+ cells give rise both to CD3- TCRalphabeta+ as well as to dimly staining CD3+ TCRalphabeta+ cells. Low pharmacologic concentrations of tacrolimus/cyclosporine (CsA) and mycophenolic acid (MPA) singly or in combination had no effect on the spontaneous proliferation of DBMC and had significantly less inhibitory activity on MLC responses of DBMC and its purified CD3+ or CD34+ subpopulations, compared with the responses of spleen cells. Moreover, the previously described regulatory effects of DBMC on the MLC responses of peripheral blood or splenic responding cells were not inhibited by these immunosuppressive drugs.

CONCLUSIONS

Taken together, these results support the notion that in vitro DBMC subpopulations, which proliferate as responding cells in co-culture with x-irradiated allogeneic cells and which cause regulatory effects when added as a third component to MLC reactions, seem to be culture-generated lymphoid cell lineage(s) progeny of CD34+ cells. This possibly includes unique CD3+ "primitive" (dimly staining) T cells, which are not as inhibited in their function by tacrolimus/CsA and MPA, as are postthymic (splenic) T cells.

Donor bone marrow potentiates the effect of tacrolimus on nonvascularized heart allograft survival: association with microchimerism and growth of donor dendritic cell progenitors from recipient bone marrow

BACKGROUND

The influence of donor hematopoietic cell microchimerism on organ allograft survival has been studied largely in vascularized transplant models. Here, we examine the impact of donor bone marrow (BM) cells administered intravenously together with transient systemic tacrolimus therapy on microchimerism, the survival of nonvascularized cardiac allografts, and growth of donor antigen-presenting cells [dendritic cells (DCs)] from recipient BM.

METHODS

Adult male C3H (H2k) mice received heterotopic heart transplants from B10 (H2b) donors in the dorsal ear pinna. They were given no further treatment, or either a short course of tacrolimus (FK506; 2 mg/kg i.p. from day 0 to day 13), unmodified donor BM cells (50x10(6) i.v. on day 0) or both treatments. Grafts were examined daily for contractile activity. Anti-donor cytotoxic T lymphocyte responses were determined in recipients' spleens. Microchimerism (IAb+ cells) was demonstrated by immunocytochemical staining of spleens, and of cells expanded from recipient BM using cytokines and culture conditions that promote the growth of DCs.

RESULTS

Tacrolimus alone significantly prolonged median heart graft survival time from 10 to 22 days (P<0.001). BM alone failed to prolong graft survival. By contrast, tacrolimus + donor BM resulted in a mean survival time of 42 days (P<0.01 compared with tacrolimus treatment alone). This marked increase in heart allograft survival was associated with reduced anti-donor cytotoxic T lymphocyte responses attributable to a nonspecific effect of tacrolimus. In addition, however, a link was observed between the beneficial effect of donor BM and comparatively large numbers of donor major histocompatibility complex class II (IAb+)-positive cells in recipients' spleens, and in cultures of granulocyte-macrophage colony-stimulating factor + interleukin-4-stimulated DCs from recipients' BM. No donor-derived cells were propagated from heart graft recipients given either tacrolimus or donor BM alone.

CONCLUSIONS

This nonvascularized organ transplant model demonstrates the positive effect on allograft survival of donor BM given at the time of transplant to transiently immunosuppressed recipients. The findings also reveal links between hematopoietic cell chimerism, the presence of donor DC progenitors in recipient BM, and organ allograft survival.

Bone marrow chimerism and transplantation tolerance

Engraftment of allogeneic or xenogeneic pluripotent hematopoietic stem cells into nonmyeloablated but immunodepleted (preconditioned) recipients can produce a state of immunological tolerance to donor and host. Host and donor hematopoietic cells entering the thymus ensure deletion of both donor- and host-reactive thymocytes. Additional mechanisms are involved in tolerance induced in recipients that are not immunodepleted. Grafting of donor thymic tissue to thymectomized recipients is an alternative approach for inducing central T cell tolerance without the requirement for engraftment of donor hematopoietic stem cells. During the past year, advances have been made in understanding both the requirements for preconditioning and the mechanisms of tolerance induction in the above transplantation models.