Localization of marginal zone macrophages is regulated by C-C chemokine ligands 21/19

The marginal zone (MZ) of the spleen is an important site for the capture of blood-borne pathogens and a gateway for lymphocytes entering the white pulp. We have recently reported that Leishmania donovani infection results in a remarkably selective loss of MZ macrophages (MZM) from the MZ. To understand the basis of this observation, we have investigated how MZM maintain their anatomical distribution in the steady state in uninfected mice. We now report that plt/plt mice, which lack functional CCL19 and CCL21, have significantly reduced numbers of MZM compared with normal C57BL/6 (B6) mice. Similarly, in B6.CD45.1-->plt/plt chimeras, donor-derived MZM were rare compared with the number observed in reciprocal plt/plt-->B6.CD45.1 chimeras. Moreover, we show that administration of pertussis toxin, an inhibitor of chemokine receptor signaling, to B6 mice results in exit of MZM from the MZ, that MZM can migrate in response to CCL19 and CCL21 in vitro, and that MZM colocalize with CD31(+)CCL21(+) endothelial cells. Collectively, these data indicate that CCL21 and, to a lesser extent, CCL19 play significant roles in the distinctive localization of MZM within the splenic MZ. Deficiency of CCL19 and CCL21, as also previously observed in mice infected with L. donovani, may thus account for the selective loss of MZM seen during this infection.