Local Modulation of Lymph Node Stroma by Sensory Neurons

Immune responses within barrier tissues are regulated, in part, by nociceptors, specialized peripheral sensory neurons that detect noxious stimuli. Previous work has shown that nociceptor ablation not only alters local responses at peripheral sites of immune challenge, but also within draining lymph nodes (LNs). The mechanism and significance of nociceptor-dependent modulation of LN homeostasis are unknown. Indeed, although sympathetic innervation of LNs is well documented, it has been unclear whether the LN parenchyma itself is innervated by sensory neurons. Here, using a combination of high-resolution imaging, retrograde viral tracing, optogenetics, and single-cell transcriptomics (scRNA-seq), we describe a sensory neuro-immune circuit that is preferentially located in the outermost cortex of skin-draining LNs. Transcriptomic profiling revealed that sensory neurons that innervate LNs are composed of at least four discrete subsets with a predominance of peptidergic nociceptors, an innervation pattern that is distinct from that in the surrounding skin. To identify potential LN-resident communication partners for LN-innervating sensory neurons, we employed scRNA-seq to generate an atlas of all murine LN cells and, based on receptor-ligand expression patterns, nominated candidate target populations among stromal and immune cells. We experimentally validated these inferred connections by comparing scRNA-seq signatures before and after selective optogenetic stimulation of LN-innervating axons. Acute neuronal activation triggered rapid transcriptional changes preferentially in endothelium and other nodal stroma cells, as well as in several innate leukocyte populations. Thus, LNs are monitored by a unique population of sensory neurons that possess profound immunomodulatory potential. We implemented a Cre-lox based viral labeling strategy that enabled reliable identification and isolation of the cell bodies of LN-innervating neurons in DRGs. Briefly, we injected a Cre-expressing recombinant adeno-associated virus, AAV2/1-Cre, a serotype with broad tropism towards DRG neurons, into the right iLN of Rosa26LSL-tdTomato/LSL-tdTomato mice carrying a Cre-dependent tdTomato reporter. Upon entry into sensory fibers, this non-replicating virus travels retrogradely to the cell body in DRGs where virally encoded Cre recombinase deletes a genomic floxed 'stop' sequence resulting in expression of tdTomato. We harvested ipsilateral T13 and L1 DRGs from injected animals to manually isolate individual tdTomato+ neurons as previously described (Hempel et al., 2007). Each isolated cell was then subjected to scRNA-Seq using the Smart-Seq2 protocol to yield a final dataset of 52 LN-innervating sensory neurons from 8 mice. As a control and reference population, we also generated scRNA-Seq libraries from 31 skin-innervating neurons from 4 Rosa26LSL-tdTomato/LSL-tdTomato mice that were retrogradely labeled by intradermal injection of AAV2/1-Cre.