Making “Sense” Of Persistent Polyclonal B Cell Lymphocytosis

Neuronal regulation of the hematopoietic stem cell niche includes elements of the sympathetic nervous system however, a role for sensory neurons has not been explored. While investigating sensory nerves in hematopoiesis, we unexpectedly found mice deficient in the neurotransmitter substance P (Tac-1^-/-) to exhibit peripheral blood lymphocytosis that is persistent from 8 weeks to at least 35 weeks of age. Flow cytometry showed the increased population to be comprised of polyclonal B-lymphocytes. Hematopathology review demonstrated abnormal peripheral blood bi-nucleated lymphocytes and enlarged splenic marginal zones. Lymph nodes were not affected. To confirm that sensory nerve disruption and not hematopoietically derived substance P was the cause, wildtype C57BL/6 mice were chemically denervated and found to exhibit similar peripheral blood lymphocytosis. By immunohistochemistry, sensory nerves synapsed at splenic marginal zones, consistent with previous reports and providing a neuro-anatomical link for the observed lymphoid hyperplasia. A human correlate to this hematological phenotype featuring long-term peripheral polyclonal B cell lymphocytosis, bi-nucleated lymphocytes and expanded splenic marginal zones is Persistent Polyclonal B-cell Lymphocytosis (PPBL), a rare disorder affecting predominantly female chronic smokers. The pathoetiology of this disorder is unknown but its strong association with smoking supports a novel hypothesis where chronic smoking leads to respiratory tract sensory nerve desensitization with possibly more systemic effects, including on the spleen, thereby mimicking the substance P deficient environment of Tac-1^-/- mice. Indeed, pulmonary function tests on chronic smokers have shown a sub-population of patients with hyporeactivity to respiratory sensory nerve stimulation, explained molecularly by increased activation thresholds. It will be interesting to determine if PPBL patients fall into this category. In Tac-1^-/- mice, further characterization of the expanded B cell population, immunoglobulin profiles, VDJ utilization and effect of neurokinin-1 receptor modulators is proposed. These results support a neuroimmune regulatory network involving sensory nerves with potential relevance in both lymphoid homeostasis and lymphoproliferative disease.