Physiological role of macrophage inflammatory protein-3α induction during maturation of intestinal macrophages

Intestinal macrophages (IMAC) are a central component in the defense of the intestinal mucosa against luminal microbes. In normal mucosa, monocytes differentiate to immunologically tolerant IMAC with a typical phenotype lacking activation markers such as CD14 and TLRs 2 and 4. CD33(+) IMAC were isolated from normal intestinal mucosa by immunomagnetic beads. A subtractive hybridization subtracting mRNA from normal IMAC from those of in vitro differentiated macrophages was performed. IMAC differentiation was studied in multicellular spheroids (MCS). Functional assays on migration of CD45R0(+) T cells were performed in MCS coculture models. Of 76 clones, 3 obtained by subtractive mRNA hybridization showed > 99% homology to mRNA of MIP-3 alpha, indicating that this chemokine is induced in IMAC compared with in vitro differentiated macrophages. MIP-3 alpha protein expression was confirmed in cryostat sections of normal intestinal mucosa by immunohistochemistry. IMAC in the lamina propria stained positive for MIP-3a. FACS of purified IMAC clearly indicated expression of MIP-3a in these cells. In the MCS-in vitro differentiation model for IMAC, MIP-3a protein expression was absent on day 1 but detectable on day 7 of coculture, demonstrating the induction of MIP-3a during differentiation of IMAC. IMAC attracted CD45R0(+) T cells to migrate into an MCS coculture model. In human mucosa, a close contact between IMAC and CD45R0(+) T cells could be demonstrated. MIP-3a is induced during the differentiation of monocytes into IMAC. Our data suggest that MIP-3a expression could be involved in the recruitment of CD45R0(+) cells into the lamina propria.