The astrocyte marker ALDH1L1 also identifies a stromal cell population in the lymph node

ALDH1L1 is widely used as a marker of astrocytes in the central nervous system (CNS), but its expression and potential roles in the periphery, particularly in lymphoid organs, remain poorly understood. Here, we found that ALDH1L1+ cells comprise approximately 5–9% of the lymph node (LN). To better understand their identity, we investigated whether these cells share characteristics with glial, immune, endothelial or stromal cell populations under homeostatic conditions. Using ALDH1L1/TdTomato reporter mice, immunofluorescence, and flow cytometry, we found that TdTomato+ cells in LNs do not coexpress canonical CNS (GFAP, ACSA-2) or peripheral glial (Sox10) markers. Similarly, using multiple approaches we found minimal overlap with T cells (CD3, TCRβ), B cells and dendritic cells (B220), myeloid cells (CD11b, Iba1), or antigen presenting cells (MHCII). To explore potential stromal associations, we assessed coexpression of TdTomato with endothelial (CD31, LYVE1) and reticular (ER-TR7) markers and found insubstantial overlap. In contrast, TdTomato+ cells showed significant colocalization with podoplanin (PDPN), a marker of fibroblastic reticular cells (FRCs). Interestingly, while many TdTomato+ cells were PDPN+, relatively few PDPN+ cells expressed TdTomato. Notably, ALDH1L1+PDPN+ cells were enriched in the paracortex and medulla but absent from the B cell follicles and subcapsular sinus. These findings suggest that while ALDH1L1 remains a reliable marker of astrocytes in the CNS, it may also delineate a distinct subset of FRCs in the LN, opening new avenues for exploring stromal cell diversity and function in lymphoid tissues.