Inflammation Induces Hematopoietic Stem Cell/Progenitor Migration and Recruitment To Lymph Nodes In Mice and Humans

Under steady-state conditions a small fraction of hematopoietic stem and progenitor cells (HSPC) circulate in the bloodstream. Circulation of HSPC follows a circadian rhythm of migration from the bone marrow (BM) to blood and possibly, lymph. The functional purpose of circulating HSPC is unclear but it has been associated with homeostatic regulation of tissue-specific reservoir of HSPC, vasculogenesis and immunosurveillance. During infectious stress, HSPC are activated by inflammation mediators and chemokines and mobilized to circulation and extramedullary tissues. Side Population (SP) cells are stem cells with pluripotent differentiation ability found in a number of adult tissues. Here we have studied whether the lymph nodes (LN) are an obligatory site of transit/homing of HSPC in inflammation. To analyze whether there is accumulation of HSPC in inflammatory lymph nodes, we first analyzed the presence of SP cells in human LN. The patient median age was 34 (range 3-89). Patient-derived LN biopsies were diagnosed by the pathologist as having reactive lymphadenitis (RL, n=35, 50.7%) or cancer (n=34, 49.3.%). Cancer diagnoses were Hodgkin’s Lymphoma (HL, n=12,17.4%), non-Hodgkin’s lymphoma (NHL, n=20, 28.9%) or adenocarcinoma (AC, n=2, 2.9%) which associate with decreasing levels of inflammation. The biopsy-derived LN cells were stained with Hoechst 33342/propidium iodide and monoclonal antibodies against CD45, CD34, CD20, CD19, CD133 and ABCG2 surface markers. Flow cytometry analysis of LN specimens identified SP cells in 82.3% of RL LN samples (1.9±1.2%; range 0-39%), 50% of HL LN biopsies (0.7±0.6%; range 0-7.7%) and 20% of NHL LN biopsies (0,03±0.01%; range 0-0.23). Interestingly, SP cells were absent from AC LN specimens. The presence of LN SP cells was significantly associated with RL versus any of the analyzed cancer groups (P<0.001). The phenotype of human SP cells in all SP-positive LN specimens was CD45⁺/CD34^-/CD19^-/CD20^-/CD133^Low/neg. ABCG2 protein expression had a strong correlation with the presence of SP cell positive samples (c²=11.8, p=0.001). To identify whether inflammation was associated to HSPC migration to LN, we analyzed the SP cells and functional myeloid progenitor cell (CFU) content of different organs in adult mice treated with lipopolysaccharide (LPS, 3 mg/Kg) or vehicle control at 6 and 12 hours post-administration. LPS treatment resulted in migration of HSPC to LN (∼13-fold), liver (∼7-fold) and kidney (∼4 fold). Circulation of HSPC in peripheral blood increased (∼8 fold) but no HSPC was found in the thoracic duct or other organs after up to 12 hours post-infusion of LPS. These data suggest the co-existence of direct migration of HSPC to peripheral blood and lymphatic vessels. Migration of HSPC to the lymph is followed by efficient LN homing/retention of HSPC. In summary, we have found that inflammation associates with HSPC mobilization and recruitment to LN in mice and humans, strongly suggesting that local inflammatory signals are crucial in the regulation of the traffic and homing of HSPC.