Leukemia-Specific Antigens Alone Are Insufficient for GVL in MHC-Matched Allogeneic Stem Cell Transplantation: An Essential Role for Minor H Antigens.

In MHC-matched allogeneic transplantation (alloSCT), donor T cells recognize minor H antigens (miHAs), which are peptide products of polymorphic genes presented by MHC. It has been presumed that leukemias must express miHAs to be targeted by alloreactive T cells. For graft-versus-leukemia (GVL) in MHC-disparate alloSCTs, tumors must express an allogeneic MHC (Reddy, 2005); however, whether miHAs need to be expressed has not been addressed. Indeed, recent work suggests that non-miHA antigens can be targeted. In alloSCT for CML, CD8 cells reactive against a non-polymorphic epitope of proteinase 3 (PR3) are amplified, a response not possible in MHC-disparate alloSCT. This response has been suggested to be augmented by the simultaneous alloresponse. If so, leukemic cells may not need to express alloantigen at all. A related question is how well antigens exclusively expressed in leukemic cells are targeted. Such antigens are derived from translocations, mutations, or from selectively overexpressed proteins, as is the case with PR3. For these antigens to be targeted by CD8 cells, the leukemia itself must have antigen presenting cell (APC) function or the antigen must be cross-presented by other APCs. To address these questions, we developed a model in which leukemias express leukemia-specific antigens and do or do not express miHAs. We have been studying GVL against a murine model of chronic phase CML (mCP-CML) induced by retroviral introduction of the human p210 cDNA into mouse bone marrow (BM) cells. mCP-CML resembles human CP-CML in that it is myeloproliferative syndrome with splenomegaly and a high white blood count consisting of maturing myeloid cells and few blasts. The retroviral constructs also express the human nerve growth factor receptor or enhanced green fluorescence protein (GFP), allowing us to track transduced cells by FACS. These leukemias therefore express two potential neoantigens: p210 and either NGFR or GFP. mCP-CML cells also overexpress murine PR3. By infecting recipient B6 or donor C3H.SW (C3) BM we created equivalent leukemias that did or did not express miHAs. In B6→B6 transplants, B6 CD8 cells did not mediate GVL against either B6 NGFR⁺ or GFP⁺ mCP-CML. Thus NGFR, GFP, PR3 and p210 are insufficient for a GVL effect. To determine whether they can be targeted when T cells are also primed by miHAs, we performed GVL experiments using the C3H.SW(H-2b)→B6(H-2b) strain pairing and compared GVL against C3 or B6 mCP-CML. To distinguish the two, we used p210/GFP and p210/NGFR to infect C3 and B6 BM respectively in one experiment and reversed the retroviruses in a second. Irradiated hosts received C3 BM, either B6 or C3 mCP-CML or a mix of both with or without 10⁶ C3 CD8 cells. Mice that did not receive donor CD8 cells died of mCP-CML regardless of which mCP-CML they received. Most recipients of B6 mCP-CML and donor CD8 cells cleared mCP-CML. In contrast, recipients of C3 CD8 cells and C3 or a mix of B6 and C3 mCP-CML died from C3 mCP-CML with similar kinetics as did mice that received no CD8 cells. Thus, concurrent immunity against miHAs did not provoke a meaningful CD8 response against leukemia-specific antigens. We also infected C3 and B6 BM with GFP/p210 to try to augment the anti-GFP response. Nonetheless, all recipients of a mix of C3 and B6 mCP-CML died without evidence of GVL. In summary, these experiments demonstrate that p210, NGFR, GFP and PR3 are insufficient for GVL regardless of whether there is a simultaneous allogeneic response and thus leukemias must express miHAs.