The chemokine receptor CCR2 maintains plasmacytoid dendritic cell homeostasis

Thymic dendritic cells (DC) play a role in central tolerance. Three thymic DC subtypes have been described: plasmacytoid DC (pDC) and two conventional DC (cDC), CD8α⁺ Sirpα^- DC and Sirpα⁺ CD8α^- cDC. Both pDC and Sirpα⁺ cDC can take up antigen in periphery and migrate into the thymus in response to chemokine signaling via CCR9 and CCR2 respectively. CCL2 is a major ligand for CCR2 and we previously showed that it was constitutively expressed in thymus, and that mice overexpressing CCL2 in thymus had reduced numbers of autoreactive T cells but elevated numbers of pDC. We have here investigated the role of CCL2-CCR2 axis in thymic pDC migration. We found that pDC expressed CCR2 at a high level and that their frequency was decreased in thymus, spleen and inguinal lymph nodes in mice lacking CCR2, but not in mice lacking CCL2. pDC migration towards the cortex or medulla within the thymus was not affected by CCL2 or CCR2 deficiency. Although some thymic progenitors expressed CCR2, this did not include those that give rise to pDC. Based on these results, we propose that CCR2 is involved in pDC homeostasis but its ligand CCL2 does not play a major role.