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Page 8 of 18 Wallace et al. J Cancer Metastasis Treat 2019;5:9 I http://dx.doi.org/10.20517/2394-4722.2019.01

complex interplay between integrins, collagen, COX-2, and SEMA7A is, thus, likely to be context-dependent,
and additional studies are necessary to understand the role of this signaling axis in mediating tumor cell
invasion and metastasis in breast cancer.

MACROPHAGES IN POSTPARTUM INVOLUTION AND PPBC
Macrophages are the phagocytic immune cells that mediate the removal of foreign pathogens, dead cells,
and debris. Classically-activated macrophages, also known as M1 macrophages, are activated in response
to pathogen-associated cytokines, most often IFN-γ and lipopolysaccharide. M1 macrophages are largely
considered to be “anti-tumor” based on their expression of the pro-inflammatory cytokines, interleukin-1
(IL-1), IL-12, TNF-α, and inducible nitric oxide synthase - all of which have been shown to oppose tumor
progression [124,125] . In contrast, Th2 family cytokines induce the maturation of alternatively-activated, or M2
macrophages, and cause their release of anti-inflammatory mediators that support tumor cell survival [21,126] .
M2 macrophages promote tumor cell growth, invasion, and metastasis, via their secretion of IL-10, TGFb,
and MMPs. Though the M1/M2 system is useful for broadly classifying macrophages, this taxonomy fails to
capture the plasticity and diversity characteristic of this cell type. For the purpose of this review, however,
we will use M1 and M2 to broadly classify anti- and pro-tumor macrophages, respectively. Multiple studies
support the role of macrophages as critical mediators of metastasis [21,127-129] . In models of gastric and breast
cancer, M2 macrophages are recruited by tumor cells, where they activate MAP-K signaling to promote
the motility of disseminating tumor cells [127] . Further, macrophages appear to be critical for the migration
of the majority of ductal carcinoma in situ cells, as only 10% are motile when macrophages are absent [130] .
The critical contribution of macrophages to tumor cell metastasis is further evidenced by studies in the
MMTV-PyMT mouse model of breast cancer, where knockout of colony stimulating factor-1 (CSF-1), a
secreted glycoprotein that induces the differentiation of hematopoietic stem cells to macrophages, correlates
with a near complete elimination of tumor cell metastasis [21,131] . Macrophages, therefore, represent a diverse
population of cells that can promote or inhibit tumor progression based on the context of their environment.

Macrophages are the primary immune cells present during mammary gland postpartum involution,
and because of their role in facilitating tumor metastasis, represent a potential contribution to the highly
metastatic nature of PPBC. Though known primarily for their phagocytic capacity, macrophages only play
a minimal role in the clearance of apoptotic cells during involution [21,132,133] . Despite their limited role in
phagocytosis, M2 macrophages are essential for the epithelial apoptosis and tissue remodeling characteristic
of postpartum involution [134] . At the peak of apoptotic cell clearance, macrophages exist at relatively low
levels, as MECs represent the primary phagocytes. At day 6 of involution, however, the peak of mammary
tissue remodeling, M2 macrophages exist at 6 times the level of those in the nulliparous mammary gland,
while classically-activated M1 macrophages remain at consistent levels throughout pregnancy, lactation,
[21]
and gland regression . F4/80, a general marker of mature mouse macrophages, marks more than the sum
of M1 and M2 macrophages during involution, suggesting there are additional macrophage populations
[20]
present in the involuting mammary gland . Our lab has recently identified a population of macrophages
[57]
that also express the lymphatic endothelial marker, podoplanin (PDPN) . In culture, SEMA7A drives the
[57]
expression of PDPN on macrophages and promotes their migration and adherence to lymphatic vessels .
Because macrophages have proven to be a critical part of the metastatic cascade by facilitating intravasation
into tumor associated blood vessels [135] , SEMA7A-mediated macrophage lymphatic mimicry may also
facilitate intravasation into lymphatic vessels, providing another explanation for the high rates of metastasis
associated with PPBC. This is further supported by the prognostic value of a combined genetic signature
of CD68, PDPN, and SEMA7A in predicting decreased distant metastasis free survival in a cohort of 600
[57]
human breast cancer cases . SEMA7A further regulates macrophages by serving as a strong activation
factor for monocytes, promoting both chemotaxis and secretion of inflammatory cytokines, in addition
to upregulation of granulocyte-macrophage CSF (GM-CSF), supporting an additional role for SEMA7A in
macrophage differentiation [136] .