Differences in Immune Cell Populations between Individuals with or without MGUS

Background: Monoclonal gammopathy of undetermined significance (MGUS) is a benign precursor of multiple myeloma (MM) with a 1% annual risk of progression. MM is incurable, necessitating a better understanding of its etiology to identify therapeutic strategies to halt its progression. Immune profile analysis of peripheral blood can enhance understanding of the leukocyte repertoire, and concomitant immunosurveillance for monoclonal plasma cells can identify early factors of progression.

Methods: We retrospectively analyzed flow cytometry-based immunophenotyping of peripheral blood samples evaluated as standard of care from individuals first seen at our center between 2007 and 2023, with either a confirmed MGUS diagnosis (study group) or negative workup (controls). Demographics, Charlson Comorbidity Index (CCI) score, and circulating leukocyte population frequencies, including exhausted/non-exhausted CD4⁺ and CD8⁺ T-cells, natural killer (NK) cells, NKT cells, antigen-activated T lymphocyte populations (AATLP), gamma delta (γδ) T cells, total CD19⁺ B cells, and memory B cells, were studied. Phenotypically defined population frequencies were expressed as percentages of total lymphocytes (% gated), percentages of a specific subset, or absolute counts per microliter (µL). Descriptive statistics were reported as means and standard deviations for continuous variables and frequencies for categorical variables. Analysis of variance assessed variations in continuous variables by race and diagnosis. Univariate and multivariate Cox models analyzed overall survival (OS) and progression-free survival (PFS), with hazard ratios (HRs) estimated. A stepwise regression model was used in the Cox multivariate model. All tests were conducted at alpha=0.05.

Results: We analyzed 247 individuals, 151 with a confirmed MGUS and 96 controls. There were 67 (44.3%) females with MGUS and 54 (56.2 %) females in the control group. The average age was 66.4 +/- 11.4 years in study group and 65.0 +/- 13.7 years in control group. Average CCI was 4.21 +/- 2.55 for study group and 3.41 +/- 2.24 for controls with females having a lower average CCI 3.42 +/- 2.35 compared to males 4.36 +/- 2.48 (p=0.006). Primary sub-diagnosis in the MGUS group was IgG kappa (30.5%). Patients in the study group had a median follow-up of 1.8 years, PFS of 8.1 years, and OS of 12.7 years. Eight individuals progressed from MGUS to SMM, 12 to MM, and two to Amyloidosis, with 10 MGUS patients dying during the reporting period. In univariate analysis for MGUS patients (previously reported), OS was significantly associated with CCI (HR=1.25), AATLP (HR=1.05), monoclonal protein (HR=2.80), non-exhausted CD4⁺ T cells (HR=0.96), and age (HR=1.09). PFS was associated with CCI score (HR=1.25), FLCR (HR=1.02), and AATLP (HR=1.03). Only CCI score was retained in the multivariate stepwise model.

The study group had significantly lower non-exhausted CD4⁺ cells (p<0.001) non-exhausted CD8⁺ T cells (p<0.001), CD19⁺ naïve B cells (p=0.011), and γδ T cells (p=0.047). Conversely, they exhibited higher AATLP (CD3⁺CD56^-CD57⁺) (p=0.011), NKT subset cells (CD3⁺CD56⁺CD57⁺) (p=0.026), and NKT cells (CD3⁺CD56⁺CD57^-) (p=0.054) compared to the control group. The study group had a higher prevalence of diabetes mellitus (p<0.001), solid tumors (p<0.001), and higher CCI scores (p<0.001).

Conclusion: We analyzed immune cell populations in peripheral blood from patients with and without MGUS. Progression to MM has been associated with loss of effector function in T, NK, and NKT cells. Previous studies in patients with MM show higher levels of NK and NKT cells and lower levels of normal polyclonal B cells and non-exhausted CD4⁺ or CD8⁺ T cells, with similar trends evidenced in our study. Novel findings from our study demonstrated higher levels of AATLP and CCI scores in the study group, compared to controls with worse survival outcomes. The AATLP data is consistent with an increase in antigen-activated cytotoxic T cells, terminally differentiated T cells, and/or T cell subsets reflecting varying stages of functional differentiation. Further elucidation of this immunology requires further investigation. These findings provide insight into early biological changes that might occur in the plasma cell disorder continuum, with prospective studies needed to gain further insight on how immune cell repertoire may contribute to prognostic factors for patients with MGUS.