Signal regulatory protein alpha (SIRP alpha) is an immunoglobulin super family protein predominantly expressed by myeloid but not lymphoid cells, and its role in lymphocyte homeostasis and function is still to be revealed. We demonstrate that mice bearing a mutant SIRP alpha lacking the cytoplasmic signaling domain (SIRP alpha MT) had an increased amount of splenic marginal zone (MZ) B cells compared to wild-type controls. Immunohistochemical analysis revealed an increased localization of MZB cells into B cell follicular areas of the white pulp in SIRP alpha MT spleens. However, we found no signs of an increased MZB cell activation level in MT mice. The immune response to T-independent antigens in vivo was slightly increased in SIRP alpha MT mice while sorted MZB from these mice responded normally to LPS in vitro. Bone marrow reconstitution experiments demonstrated that the MZB cell phenotype of SIRP alpha MT mice was due to lack of SIRP alpha signaling in non-hematopoietic cells. In contrast, MZ retention of MZ macrophages required hematopoietic SIRP alpha, while normal distribution of metallophilic macrophages required nonhematopoietic SIRP alpha signaling. In summary, these data identified SIRP alpha signaling in nonhematopoietic cells to play an important role in regulating the numbers and positioning MZB cell in the spleen. (C) 2015 Elsevier Inc. All rights reserved.