Date Published: January 18, 2012
Publisher: Hindawi Publishing Corporation
Author(s): John L. Reagan, Loren D. Fast, Eric S. Winer, Howard Safran, James N. Butera, Peter J. Quesenberry.
Much of the therapeutic benefit of allogeneic transplant is by a graft versus tumor effect. Further data shows that transplant engraftment is not dependant on myeloablation, instead relying on quantitative competition between donor and host cells. In the clinical setting, engraftment by competition alone is not feasible due to the need for large numbers of infused cells. Instead, low-level host irradiation has proven to be an effective engraftment strategy that is stem cell toxic but not myeloablative. The above observations served as the foundation for clinical trials utilizing allogeneic matched and haploidentical peripheral blood stem cell infusions with minimal conditioning in patients with refractory malignancies. Although engraftment was transient or not apparent, there were compelling responses in a heavily pretreated patient population that appear to result from the breaking of tumor immune tolerance by the host through the actions of IFNγ, invariant NK T cells, CD8 T cells, NK cells, or antigen presenting cells.
Allogeneic marrow transplantation exerts much of its therapeutic effect through graft killing of tumor cells. This was established in studies of the effect of donor lymphocyte infusions on relapsed chronic myelocytic leukemia (CML) in marrow transplant patients .
These studies suggested that cellular approaches without toxic myeloablative therapy might be effective treatment for many marrow malignancies. General dogma had been that myeloablative treatment was needed to open space so that marrow cells could engraft. However, Micklem et al., in 1968, demonstrated that engraftment into nonirradiated hosts was feasible, obtaining up to 8.5% T6T6 donor cells in CBA host marrow at 3 months after transplantation of 20 million marrow cells from normal T6T6 donors . This work was extended by others [13–15].
These data formed a base for engraftment into non-myeloablated mice. However, engraftment into non-myeloablated mice required large number of marrow cells, which would be difficult to obtain in a clinical setting. Accordingly, we investigated whether minimal myeloablation with low doses of irradiation would be an effective engraftment strategy . In these studies, we demonstrated that exposure of BALB/c mice to doses of irradiation that cause minimal myeloablation (50–100 cGy) gave high levels of donor chimerism, such that relatively small numbers of marrow cells (10–40 million) can give donor chimerism in the 40–100% range. These doses of irradiation turned out to be minimally myeloablative but quite stem cell toxic. Engraftable stem cells measured at 8 weeks after engraftment from mice exposed to 100 cGy whole body irradiation were reduced to 8.6 ± 3% of marrow from nonirradiated mice. At 6 months, the reduction was still present, 21 ± 7% . These data provided us with a stem cell toxic nonmyeloablative approach for therapeutic transplantation. Engraftment with reduced intensity conditioning is summarized in List 2.
However, it was not clear if this would hold for allogeneic engraftment. Accordingly, we investigated engraftment into non or minimally myeloablated allogeneic mice. We felt that low-dose irradiation might avoid the cytokine storm which appeared to be involved in GVHD, and that relatively high levels of marrow cells might overcome rejection. However, we found that we could not obtain engraftment using 100–300 cGy and 40 million cells in H-2 mismatched B6.SJL to BALB/c marrow transplants. Clearly, immune barriers existed.
These studies plus the evolving studies on the impact of donor lymphocyte infusions in clinical transplant set the stage for human trials using allogeneic or haploidentical peripheral blood infusions in patients with refractory cancers. Patients with refractory cancers were treated with 100 cGy total body irradiation followed by infusion of nonmobilized apheresed allogeneic peripheral blood cells. Twenty-five patients were enrolled . Transplants were with either HLA matched or 1/6 mismatched, one antigen mismatched family donors, or 4–6/6 antigen matched umbilical cord blood donor cells. Seven patients with solid tumors received a sibling transplant and 6 received a cord blood transplant; none achieved donor chimerism but 1 treated at the higher-dose level of 1 × 108 CD3+ cells/kg had a transient nodal response.
The underlying mechanism of cellular therapy efficacy is only recently coming to light and is thought to include everything from the growth factors employed for cell collection to the actual composition of the cells themselves. Clearly T cells are involved in this process; however, neither the bystander killing effect nor the T cell receptor cross reactivity to allogeneic antigens with tumor antigens is thought to play a role. Central to the development of a graft versus leukemia or host versus leukemia response is the underlying regulation between the development of a TH1 response and a graft or host versus tumor effect (and with it the potential for acute or chronic GVHD) and a TH2 response that allows for tolerance of the graft and prevents GVHD. Clinical trials suggest that an initial engraftment of donor cells followed by the host rejection of this engraftment is essential to developing a host versus tumor response. Based on this concept, Sykes and colleagues developed a mixed chimeric bone marrow murine model to more fully understand the mechanism of this response. The mixed chimera developed represents the initial infusion and partial engraftment of haploidentical cells. They are then able to model the rejection of these haploidentical cells through the infusion of recipient murine leukocytes (RLI). RLI infusion then induces a host versus graft (HVG) reaction that results in an antitumor response by the host’s immune system to malignancies present within the host which they have been able to further characterize. This response requires a complex interaction between interferon-γ (IFN-γ), CD8 T cells, invariant NK-T cells (iNKT), NK cells, and antigen presenting cells.
As outlined above, cell infusional treatment of leukemia and lymphoma is not completely dependent on graft versus tumor/leukemia effect. Rather, rejection of the graft itself appears to reeducate the host’s immune system to recognize the tumor, thereby creating a host versus tumor effect. This method is not without its side effects in the form of a cytokine storm involving IFN-γ but does bypass the morbidity and mortality that develop from graft versus host disease. An additional advantage of non-engraftment haploidentical transplantation over allogeneic transplantation is the availability of donors. Family members are readily identified as potential donors making this a readily available treatment modality. The overall keys to the future of nonengraftment haploidentical transplantation reside in better understanding of the degree of chimerism, if any, necessary for a reaction as well as the timing and role of host effector cell stimulation. The potential result is the ability to harness the host’s immune system in order to provide an effective therapeutic modality to eradicate malignancy. Future studies should help decipher the underlying interaction that occurs between host and donor cells. Key questions that remain are the amount and type of conditioning regimen as well as the degree of antigenic mismatch required to stimulate a host versus tumor response in vitro and in vivo. Potential further clinical trials could focus on decreasing or eliminating radiation or chemotherapy conditioning in addition to examining the effects of complete HLA mismatch in order to further remove any chance of engraftment and with it GvHD development. Moreover, future trials may also explore the role of multiple cellular infusions spaced out in a treatment plan to possibly elicit a more robust response.