Long-Term Survivors after Stem Cell Transplantation in Multiple Myeloma: Bone Marrow Minimal Residual Disease, PET/CT and Immunological Status

Introduction: Multiple myeloma (MM) is considered an incurable disease. Best results are obtained with autologous stem cell transplantation (ASCT). The achievement of complete remission (CR) is the crucial step for a prolonged survival. Unfortunately, even with the use of new regimens, the vast majority of patients will ultimately relapse. In fact, with conventional chemotherapy, ASCT or allogeneic stem-cell transplantation (alloSCT), only 1‰, 10% and 15%, respectively are alive and in continued CR beyond 10 years in the best circumstances. The aim of the present research was to evaluate the proportion of patients in long-term CR after stem-cell transplantation, describing the different aspects related to minimal residual disease (MRD) and immunological correlative studies.

Methods: We analyzed the medical records of 160 patients who underwent single melphalan-based ASCT from March 31^th 1994 to December 31^th 2010, allowing at least 5 years of follow up from diagnosis (median follow-up of 8 years), as well as 58 patients who had received an alloSCT over a 28-year period.

Bone marrow (BM) samples from the alive CR patients beyond 5 years were analyzed to assess MRD using multiparameter flow cytometry (MFC) with Euroflow 8-color consensus panel. Serum free-light chains (FLC) and immunoglobulin heavy/light chains (HLC) (The Binding Site) were assayed by immunonephelometry. A whole-body PET/CT (Siemens) was also performed to evaluate extramedullary and bone disease. Furthermore, a detailed panel of surface cell markers by MFC was also developed to evaluate B, T and NK cell compartments, among other immune cell subpopulations in BM and/or peripheral blood.

Results: Twenty patients (20/160; 12.5%) were alive and in continuous CR after ASCT beyond 5 years; nine (5.7%) remained in CR for more than 10 years. In the alloSCT group, 6 out 58 patients (10.3%) were in CR beyond 5 years (3 of them for more than 10 years). Remarkably, two patients were in CR after 20 and 28 years after ASCT and alloSCT, respectively.

Fourteen long survivors after ASCT (70%) had received maintenance therapy. These regimens were mainly interferon (8 patients), followed by thalidomide (3) or thalidomide plus bortezomib (3).

None patient showed increased uptake of FDG by PET/CT suspicious of MM activity, neither in bone lesions nor previous plasmacytomas. However, in the vast majority of patients (16/22; 72.7%) there were stable lytic lesions, usually with sclerotic borders. Unspecific findings, particularly lymph nodes with unspecific FDG uptake, were described in 28.6%.

Fourteen out of 20 patients after ASCT (70%) and 2 after alloSCT (33.3%) showed oligoclonal bands (OB) in serum and/or urine, differents from the original monoclonal paraprotein by immunofixation. Among the 22 evaluated patients, all but 2 were negative by MFC for MRD in BM. These two patients were in stable CR, with evidence of OB in serum, considering the clone found unrelated to the original one. The aberrant plasma cell population (CD38+/++, CD56+, CD27-, CD19-, CD45-, CD81-, CD117+) was found to a frequency of 0.09% and 0.004% BM cells. Other 4 patients showed a population of plasma cells with some aberrant markers expression; two of them also showed OB.

All patients showed normal serum FLC ratio except one patient after alloSCT. Levels of IgG and IgA HLC were also normal, except in one patient each. Six patients had higher IgM HLC ratio than normal, all of them showing OB.

We also investigated whether T cell clonal expansion could be also involved. Indeed, patients with OB showed a higher diversity of TCR Vb over-represented, preferentially 13.1 and 17, suggesting a potential oligoclonal expansion of the T cell response. Characterization of TCR repertoire of the long survivors also indicated a higher involvement of the CD8+ compared to CD4+ T cells. In BM, the expression of the co-inhibitory receptor PD-1 on CD8+ T cells was elevated in active MM compared to MGUS and long survivors suggesting that exhausted PD1+CD8+ T cells increase in active MM and recover to normal levels in long survivors.

Conclusions: There are a small proportion of long-term survivors after SCT in MM. MRD studies are negatives, but some false positives could be observed by MFC when OB are present. PET invariably showed metabolic CR with stable bone lesions. Our results support that oligoclonal humoral and cellular responses may play an important role in the disease control over the years.