Splenic B Cell Lymphoma Innate Immune Cells: Detection of Red Versus White Pulp Lymphoma Differences in NK Cell Populations By High-Parameter Flow Cytometry

Introduction

Splenic B-cell lymphomas, (SLs, total incidence ~1:100,000/year) comprise four distinct indolent B-cell non-Hodgkin lymphomas. Three predominantly involve the red pulp of the spleen (classical hairy cell leukemia (HCL), hairy cell leukemia variant (HCLv) and splenic diffuse red pulp small B-cell lymphoma (SDRPL)) and the fourth predominantly involves the white pulp (splenic marginal zone lymphoma (SMZL)). HCL is usually diagnosed by detection of an activation mutation in BRAF and responds well to treatment. We focused our study on the remaining three diseases, which have overlapping clinical presentations with only subtle differences in their morphology and phenotypes, but have considerable differences in prognosis, response to treatment and outcome. The white pulp lymphoma SMZL generally has good treatment responses and longer survival, whereas the red pulp SLs, HCLv and SDRPL are less responsive to treatments with shorter survival.

Cells of the tumor microenvironment (TME) may have distinct characteristics between these three diseases. We hypothesized that TME innate immune cells from red pulp SLs (HCLv, and SDRPL) would be similar to each other and distinct from the white pulp SL, SMZL. In this study, we report that natural killer (NK) cells, which are important innate immune cells critical for immune surveillance, vary in proportions among red and white pulp SLs.

Methods

We studied splenectomy specimens from patients with SMZL (n=8), HCLv (n=8), SDRPL (n=7), and controls (healthy, post-trauma, n=8). Cryo-preserved bulk splenic suspension cell isolates from fresh (<4 hour) surgical specimens and intact formalin-fixed paraffin embedded (FFPE) tissue sections were used. Human specimen collection and usage was conducted with written informed consent after approval of the Institutional Research Subjects Review Board.

FFPE tissue sections were studied by immunohistochemical (IHC) staining and splenic suspension cells were analyzed by high-parameter fluorescence flow cytometry using a Cytek Aurora spectral flow cytometer with a 25-color myeloid and a 28-color NK/B-cell panel.

Results

IHC characterization showed an extensive network of innate immune cells as reflected by CD163⁺ macrophages.

Using high-parameter flow cytometry, we focused on innate immune cells in the non-B- non-T-cell (CD3^-CD19^-) population. The average proportions of this population were: NK cells 25-36%, monocyte/macrophages 20-34%, dendritic cells 5-11%, and granulocytes 2%. Among the patient groups, the median percentage of NK cells was greater for controls (34%) and SMZL (35%) as compared to red pulp lymphomas (HCLv (28%), SDRPL (24%)). Next, we examined NK subtypes: 1) cytotoxic NK cells (CD56⁺CD16⁺), 2) cytokine-secreting weakly cytotoxic NK cells (CD56⁺CD16^-), and 3) undefined NK cells (CD56^-CD16⁺) as a median percentage of total NK cells. The major subtype for control and SMZL was CD56⁺CD16^- with values of 58 and 44%, respectively. This subtype comprises only 38%-40% of red pulp SLs. Proportionally the CD56⁺CD16⁺ subtype was the largest for red pulp SLs at 52-57% as compared to 30% for control and 38% for SMZL. The remaining CD56^-CD16⁺ NK cell subtype was generally higher among red pulp SLs (HCLv (9%), SDRPL (12%)) as compared to control (6%) and SMZL (4%) patients. The increase in cytotoxic CD56⁺CD16⁺ subtype among red pulp SLs was confirmed by measuring the percentage of granzymeB⁺perforin⁺ cells among mature cytotoxic NK cells (CD56⁺CD16⁺/ CD57⁺NKG2a^-). SLs had higher proportions of this NK phenotype (78-86% compared to control 69%) and red pulp SLs had the most granzymeB⁺perforin⁺ proportions with 82 and 86% for HCLv and SDRPL, respectively.

Discussion

In summary, spleens from patients with SMZL had similar percentages of total NK cells to controls, with lower percentages in red pulp SLs. All SLs had a higher percentage of cytotoxic NK cells than control, with the red pulp SLs having the most. These data suggest that monoclonal antibody treatments targeting SL antigen, such as anti-CD20, may be effective by activating cytotoxic NK cell to target lymphoma cells by antibody-dependent cellular cytotoxicity. Thus, uncovering the characteristics of the TME may aid the development of non-invasive diagnostic procedures and/or precision therapies.