Characterization of Early Natural Killer Cell Reconstitution Following Autologous Transplantation in Multiple Myeloma.
Authors: Benson Don , Bakan Courtney E , Alghothani Lana , Hofmeister Craig C. , Efebera Yvonne A , Smith Megan K , Greenfield Carli N , Andre Pascale , Squiban Patrick , Romagne Francois , Caligiuri Michael A. , Devine Steven M 
Published: October 2019

Abstract
Abstract 4641

Background
Despite the advent of novel therapies, multiple myeloma (MM) remains incurable and high-dose chemotherapy (HDC) with autologous stem cell transplantation (SCT) remains an important therapy for this incurable malignancy. Prior research has suggested that early lymphocyte immune reconstitution following HDC/SCT may be associated with improved outcomes; however, little is known regarding the biologic mechanisms underlying this phenomenon. A potent natural killer (NK) cell versus MM effect has also been demonstrated by our group and others previously, and NK cells are the first lymphocyte subset to recover following HDC/SCT. Herein, we characterize the nature of this recovery to suggest that the early post-transplant period may be an especially favorable environment in which to augment the NK cell versus MM effect to improve outcomes for patients with MM undergoing HDC/SCT.

Methods
With informed consent and under institutional review board approval, peripheral blood samples from patients with multiple myeloma undergoing HDC/SCT were collected on the day of first peripheral white cell count > 1500/uL post- HDC/SCT. All patients received intravenous melphalan (200mg/m2) as the preparative regimen. Mononuclear cells were isolated and, by flow cytometry, CD56(+),CD3(-) NK cells were comprehensively immunophenotyped for expression of inhibitory and activating ligands. NK cell activation and cytotoxicity were assessed by ELISPOT and chromium release assays. Clinical outcome was assessed as a function of early NK cell recovery.

Results
11 patients (median age 57, 8 male, 3 female) with a median of 1 prior regimen to transplant (range 1-9) have been assessed to date. 1 patient was mobilized with AMD-3100, 3 had cyclophosphamide mobilizations and 7 were mobilized with neupogen alone. The average CD34(+) cell dose received was 5.22 (+/- SD 2.39). The median day of peripheral white cell recovery was day +12 (range 11-18). The median absolute lymphocyte count on sample procurement day was 410 (range 340 – 820), of which an average of 37% (range 13 – 70) were NK cells. Although total white count on procurement day correlated with CD34(+) cell dose (p = 0.02), neither absolute lymphocyte count or NK cell count correlated significantly with CD34(+) cell dose and mobilization strategy did not affect NK cell count. NK cell reconstitution correlated with best response to HDC/SCT (p=0.02), the average NK cell count for patients ultimately achieving CR was 389 (+/- 131) as compared to those achieving PR or less (171 +/- 101). Interestingly, nearly half of reconstituting NK cells (46% +/- SD 20) expressed CD117 (c-kit receptor), a marker shown previously to be associated with earlier stages of NK cell development. Inhibitory KIR (CD158a on 36% +/-17 and CD158b on 36% +/- 24) and NKG2A (76% +/- 10) expression also seemed restricted to the CD117(+) NK cells. This population of NK cells also displayed NKG2D (25% +/- 22) and CD16 (73% +/-8). Reconstituting NK cells were capable of interferon gamma production against MM tumor targets similar to NK cells isolated from healthy donors (E:T = 20:1, 956 +/- 98 spots/well versus 896 +/-85, p = n/s). Granzyme B production against MM tumor targets also compared to normal controls (E:T = 20:1, 547 +/- 66 versus 573 +/- 50, p= n/s). Moreover, reconstituting NK cells showed enhanced cytotoxicity against MM tumor targets when pre-treated with an inhibitory KIR blocking antibody (IPH2101/1-7F9) similar to healthy donor NK cells.

Discussion
Taken together, we provide quantitative and qualitative data to suggest that early NK cell recovery post-SCT may contribute to outcome in patients with MM. This may be an optimal period to implement immunomodulatory strategies to augment NK cell function to improve outcomes for patients with MM undergoing HDC/SCT. Reconstituting NK cells, although exhibiting a relatively unique immunophenotype compared to mature NK cells in healthy, normal donors, appear to retain properties of activation and cytotoxicity. Given the favorable effector:target ratio in vivo following SCT, these results support further research into immunotherapeutic manipulation of the reconstituting NK cell clone to improve efficacy of HDC/SCT in patients with MM.

Disclosures:
Andre: Innate Pharma: Employment. Squiban:Innate Pharma: Employment. Romagne:Innate Pharma: Employment.