Background: Most studies in immune reconstitution after allogeneic hematologic stem cell transplantation (alloHSCT) focus on adaptive immunity, particularly in lymphocyte recovery. Classical monocytes (cMo) are part of the innate immunity, playing an important role in the defense against fungal and bacterial infections. The innate and adaptive immunity are closely related, and non-classical monocytes (ncMo) participate in this connection, regulating T cell response. However, the role of these cells in the graft versus tumor effect and graft versus host disease (GVHD) is still unclear.
Objectives: The aim of the study was to investigate cMo and ncMo reconstitution after alloHSCT and a possible impact of these cells subsets on acute and chronic GVHD, relapse, non-relapse related mortality (NRM) and overall survival (OS).
Patients and Methods: This is a retrospective analysis of 77 patients who underwent alloHSCT in 4 different transplant centers in Brazil. Peripheral blood samples were collected from patients at neutrophil engraftment (NE) and 3, 7, 14, 21, and 42 days after NE. The cMo (DR+, CD11c+, CD14+, CD16-) and ncMo (DR+, CD11c+, CD14-, CD16+) were classified and analyzed by 8-color multiparametric flow cytometry (FACSCanto II). In order to analyze the possible impact of duration and severity of monocytopenia after transplant, we calculated then the 'mono-index' and the 'ncMo-index', based on the area over the curve (AOC) of absolute cMO counts (AMC) and absolute ncMo counts, respectively, considering a horizontal line of AMC 0.5x10e9/L as cut-off value for both subpopulations.
Results: The medium follow up was 82 months. Out of 77 patients, 46 (60%) were male, and the median age was 17 years (range: 1-74). The stem cell source was bone marrow in 36 (47%), cord blood in 23 (30%), and peripheral blood in 18 (23%); and donors were matched unrelated in 61 (79%), and matched related in 16 (21%) cases. Most patients received myeloablative conditioning (n=47, 61%); and acute leukemia was the most common diagnosis (72%). A total of 40 (52%) patients received total body irradiation (TBI), and 27 (35%) patients received antithymocyte globulin (ATG). We calculated the possible impact of cMo at all timepoints and found no differences in the main outcomes between patients with lower or higher counts. We then analyzed the AOC of all cMo counts (the mono-index). A higher index represents a more severe deficiency of cMo. We then used the receiver operating characteristic (ROC) curve to define the cut-off value of 14234.4 cells/days x mm3 to discriminate patients as cMo deficient (n=31, 40%) or non-deficient (n=46, 60%). Non relapse mortality (NRM) at 6 years was higher in cMo deficient patients calculated by the mono-index (40% vs. 16% for non-deficient, p=0.02, Figure 1). Deficient patients also had worse OS at 6 years (58% vs. 37% for non-deficient, p=0.03, Figure 2). In multivariate analyses, cMo deficiency calculated by the mono-index remained significantly associated with NRM (hazard ratio 3.07, 95% confidence interval) and OS (HR 2.05, 95%CI 1.20-7.85). We observed no impact of mono-index on acute or chronic GVHD or relapse. We observed no impact of the ncMo counts at all timepoints or the ncMo-index in any of the analyzed outcomes.
Conclusion: cMo deficiency calculated by the mono-index can predict mortality in alloHSCT. Mono-Index is an inexpensive, accessible and easy to perform tool that might help clinicians in predicting unfavorable outcomes. Further studies are needed to confirm these findings and to test how different transplant strategies or therapeutic changes might interfere with monocytes recovery in alloHSCT.
No relevant conflicts of interest to declare.
