The concept of the mononuclear phagocyte system (MPS) proposed by van Furth et al. 
has been constantly revisited. According to the MPS concept, all macrophages are derived from monocytes. However, according to phylogenetic and ontogenetic points of view, macrophages emerge before the development of monocytic cells. With regard to ontogeny, primitive and fetal macrophages first develop in the yolk sac where monocytes are not detected, and this indicates that in early hematopoiesis, macrophages arise bypassing monocyte development 
. In addition, hematopoietic stem cells have been shown to differentiate without passing through the developmental stages of monocytic cells. Data here described question the MPS concept, pointing out that the development and differentiation of macrophages occurs not in a single cell lineage, but possibly through multiple pathways.
The differentiation pathway of macrophages via monocytes is supported by M-CSF and inflammatory signals. It has been demonstrated that in op/op(−/−)
mice, this differentiation pathway is completely blocked. However, results show that this blockage does not occur in a number of primitive macrophages or in immature tissue macrophages suggesting the heterogeneity of macrophage origin 
Reports in the literature support the presence of M-CSF-independent macrophages in op/op(−/−)
mice that have differentiated not from monocytes but from early (unidentified) progenitors 
. M-CSF-independent macrophages are considered immature macrophages, and it is thought that they are present in various tissues in the absence of inflammatory signals or that they rapidly migrate in response to an inflammatory stimulus. Furthermore, M-CSF-independent macrophages are described as small round cells characterized by an ultrastructure containing a low intracellular organelle content, particularly lysosomal granules 
. It is interesting to note the similarities between the morphology of M-CSF-independent macrophages and that described for B-1 cells 
. Based on the data reported in this study, we speculate that the M-CSF-independent macrophages described in the literature could be derived from B-1 cells.
In this study, we demonstrate that the deficiency of the cfms
gene in op/op(−/−)
mice does not influence the peritoneal B-1 cell population, in the same way as it affects macrophage population. Furthermore, when compared with normal littermates, B-1 cell proliferation levels increase with LPS stimulation in op/op(−/−)
mice. Intriguingly, intraperitoneal LPS stimulation resulted in a significant enlargement in the macrophage population in op/op(−/−)
mice. As discussed above, it is reasonable to suggest two origins for macrophages, those derived from monocytes and those “macrophages” derived from B-1 cells, which are the predominant macrophages found in op/op(−/−)
mice. This hypothesis is supported by the Ig gene rearrangement (VH11) found in “macrophages” from op/op(−/−)
mice and their normal littermates. Consistent with our previous data, B-1 cell-derived phagocytes contain the VH11 or VH12 Ig gene rearrangement, which is typical of B-1 cells 
. Importantly, the levels of VH11 detected in “macrophages” from op/op(−/−)
mice after LPS stimulation is higher in comparison with the op/op(+/+)
levels, indicating that “macrophages” from op/op(−/−)
mice are predominantly formed by B-1 cell-derived phagocytes. The transfer of B-1 CFSE+
cells to the peritoneal cavity of BALB/Xid
mice and the presence of CD11b+
macrophages support this hypothesis.
Our data also confirm that following an inflammatory stimulus, the macrophage population in the peritoneal cavity increases not only due to monocyte migration but also because of proliferation of resident peritoneal cells. As indicated by Jenkins et al.
, pleural macrophages proliferate in situ
in response to Th2 inflammatory signals. The study proposes that macrophage proliferation in situ
is an alternative mechanism of inflammation that increases the phagocyte population for the performance of critical functions, such as parasite sequestration or wound repair, in the absence of potentially damaging cell recruitment. However, we demonstrated that increase in macrophage population after LPS stimulation is a consequence of a proliferation in one of the resident peritoneal cells, other than monocytes. Moreover, the transfer of B-1 CFSE+
cells to BALB/Xid
mice clearly demonstrated that LPS treatment leads to B-1 cell proliferation and differentiation into phagocytes. Based on this data, we question if monocyte-derived macrophages proliferate in vivo
. Furthermore, we reinforced our hypothesis by considering more than one origin for peritoneal macrophages. The definitive role of B-1CDPs in the inflammatory response and their function as macrophages is under investigation.
The discovery that B-1b cells can be obtained from adherent mouse peritoneal cell cultures and that these cells spontaneously differentiate into mononuclear phagocytes that are unrelated to monocytes introduced a new candidate for the mononuclear phagocytic system 
. It is important to note that B-1b cells migrate from the peritoneal cavity to non-specific inflammatory lesion sites just as monocytes migrate from the bone marrow. Recent studies in our laboratory support the existence of B-1 cell-derived phagocytes in vivo
. These cells ingest apoptotic bodies and E. coli
bacteria in vivo
, although less effectively than macrophages. Other authors also demonstrated the phagocytic and microbicidal ability of peritoneal B-1 cells 
. Consistent with these data, Borrello and Phipps 
demonstrated that when splenic B-1a cells were co-cultivated with fibroblasts, they became phagocytes, which they termed B/macrophages 
. These authors proposed that in addition to the monocytic origin of macrophages as defined by the concept of the mononuclear phagocytic system, macrophages could also originate from B-1 cells.
It has been demonstrated that the pre-B cell line J13 differentiates into CD5-positive “macrophages” after culturing for 1 month in the presence of GM-CSF 
. Furthermore, the authors analyzed different mouse strains, including mev/mev
mice, to detect CD5-positive “macrophages” in vivo
. CD5-positive macrophages were observed only in the mev/mev
mice. Importantly, the mev/mev
mice are deficient in B and T cells but have B-1 cells in abundance, which is in contrast to other B-1 cell-deficient strains. In this study, we observed that clodronate treatment abolished macrophage cells in the peritoneal cavity of BALB/c and BALB/Xid
mice, even after LPS stimulation. The observation that the “macrophage” population was present in the peritoneal cavity after clodronate treatment only when B-1 cells were transferred to BALB/Xid
mice supports the hypothesis that LPS-elicited macrophages originate not only from monocyte-derived macrophages but also from B-1 cell derived phagocytes. Additionally, experiments using fetal liver or bone marrow transplants in scid
mice have confirmed that CD5-positive “macrophages” develop from B-1 cells and are detected in milky spots and the peritoneal cavity 
Plytycz and Seljelid 
proposed that B-1 cells are ‘living fossils’, and suggested they are primordial cells in the phylogenesis of the hematopoietic system. Extensive adoptive transfer studies 
suggest that the progenitors of B-1a cells reside in the fetal omentum and fetal liver and are missing in the bone marrow of adult mice. Furthermore, B-1 cells appear in the para-aortic splanchnopleura of 9.5 (dpc) embryo mice. A “primitive” macrophage population that is derived from the yolk sac and is not related to monocytes has been detected in mouse embryos (7 dpc). Monocytes appear in the embryonic liver on day 11 (pc) when definitive hematopoiesis occurs 
. Coincidently, B-1 cells appear in mammals at a similar stage of development, before the hematopoietic system develops.
The biphenotypic B/macrophage lineage precursor present in mammals suggests a close evolutionary relationship between B cells and monocyte-derived macrophages, implying a possible common phylogenetic predecessor with attributes of both cell types. Reinforcing this hypothesis is the demonstration by Li and colleagues 
that lymphocytes in teleostean fish, a primitive animal, are biphenotypic.
The data presented here demonstrate a relationship between B-1CDPs and M-CSF-independent macrophages in op/op(−/−)
mice. The hypothesis that B-1 cell-derived phagocytes are related to the M-CSF-independent macrophage population is also supported by the absence of the M-CSF receptor in B-1 cells 
. Despite the fact that B-1CDPs acquire MCSF-R after differentiation in vitro
, the absence of the receptor in B-1 cells suggests that M-CSF do not act as an important factor to induce differentiation of B-1 cells into phagocytes. However, the role of M-CSF signaling in B-1 cell differentiation into phagocytes needs further investigation.
Furthermore, depletion of macrophages by clodronate treatment in a normal mouse strain (BALB/c) confirms the participation of B-1 cell-derived phagocytes as a component of the peritoneal macrophage population. Moreover, detection of CFSE+ macrophages in the peritoneal cavity of BALB/Xid mice after the transfer of B-1 CFSE+ cells reinforces the hypothesis that B-1 cells differentiate into phagocytes in vivo after LPS stimulation. The current knowledge about B-1 cells and their ability to differentiate into phagocyte, incite a critical visit to MPS concept, in order to reconsider that vertebrates have at least two professional mononuclear phagocyte systems: one derived from monocytes and the other from B-1 cells.