Macrophages are white blood cells that accumulate in tumors, where they aid cancer progression.
Now scientists have identified a surface protein found only on the macrophages residing in tumors, exposing a target for precise tumor treatments.
Most tumors contain macrophages, a type of immune cells, that aid tumor growth through a number of mechanisms.
Tumor associated macrophages (TAMs) act to stimulate growth of new blood vessels, remodel the extracellular matrix to promote metastasis, and enhance drug resistance.
Despite the role of TAMs in aiding tumor growth, because of their high concentration in tumors, researchers have sought ways to exploit these cells for targeting tumor therapies.
A major barrier to achieve this goal, though, is how to distinguish TAMs from macrophages in normal organs. Now, NIBIB-funded Hongbo Pang, Ph.D., Assistant Professor, College of Pharmacy at the University of Minnesota, and his collaborators have identified a protein found only on the surface of TAMs.
The work, which aims to guide specific targeting of tumor therapies, is reported in the May issue of the Journal of Controlled Release.
“Delivering drugs to tumors, while avoiding exposure to healthy tissues, is a central goal in cancer treatment'” says David Rampulla, Ph.D., director of the NIBIB program in Delivery Systems and Devices for Drugs and Biologics.
“Identifying a cell surface protein specific to only macrophages in tumors and not healthy tissues is an exciting finding with the real possibility of improving the specificity and potency of therapies for a wide range of cancers.”
Pang and his colleagues previously used a technique called phage display to identify a marker found only on the surface of TAMs.
Phage display involves putting thousands of random pieces of DNA into phages, which are viruses that infect and grow in bacteria.
The piece of DNA in each phage makes a peptide, a small piece of a protein, that is displayed on the surface of the phage.
The result is an extensive library of random peptides on the surface of the phages.
The library of phages was then exposed to a macrophage cell line, which led to the identification of a macrophage-binding peptide dubbed CRV.
In this study, Dr. Pang and his team injected CRV into mice carrying a variety of tumor types. CRV successfully homed and bound to TAMs in the tumor tissue while avoiding surrounding healthy tissues.
To bind the TAMs, CRV had to move from the tumor’s blood vessels into the tumor tissue itself – a process called extravasation.
The rapid extravasation of CRV to reach TAMs was an extremely encouraging sign that this peptide has the potential to carry therapeutic cargo into solid tumors.
The team validated that CRV binds to a receptor on the surface of TAMs called RXRB. Further analysis confirmed that RXRB is not found on the surface of macrophages in normal organs and is therefore a distinct marker found only on TAMs.
To test the idea of using the system to deliver anti-tumor drugs, the team attached CRV to a nanoparticle, which could potentially carry chemotherapy drugs to TAMs. The CRV-nanoparticle was injected into mice carrying mouse mammary tumors. CRV enhanced the delivery of the nanoparticle into solid tumors.
“The results demonstrate that we have defined a potentially novel target on TAMs for improving TAM-based cancer therapy,” says Pang.
“This opens the possibility for a number of therapies that target TAMs, ranging from highly specific delivery of chemotherapy to tumors, to the development of TAM-binding molecules that could potentially reverse TAMs from being tumor promoters to potent anti-tumor weapons.”
Tumors are complex tissues where cancer cells maintain intricate interactions with their surrounding stroma.
Important components of the tumor stroma include macrophages, which are intimately involved in tumor rejection, promotion, and metastasis.
In some cases, macrophages can comprise up to 50% of the tumor mass, and their abundance is associated with a poor clinical outcome in most cancers.
Tumor-associated macrophages (TAMs) promote tumor growth by facilitating angiogenesis, immunosuppression, and inflammation, and can also influence tumor relapse after conventional anticancer therapies.
Strategies aimed at targeting TAMs have shown great promise in mouse models, and a number of these agents are currently under clinical investigation.
Here, we review current understanding of how TAMs regulate tumor progression and provide a comprehensive update of therapies targeting macrophages for the treatment of solid cancers.
We also evaluate the contribution of TAMs in moderating the effectiveness of different anticancer treatment modalities and reflect on the challenges that need to be addressed to successfully incorporate the targeting of TAMs into current anticancer regimens.
The Ontogeny of TAMs
Macrophages are required to maintain homeostasis in the organs they occupy.
Given the specific needs of each tissue microenvironment, there are many different types of macrophages with morphologically and functionally distinct characteristics.
Prototypical examples include liver Kupffer cells, brain microglia, and lung alveolar macrophages, which together reflect the versatility of the mononuclear phagocytic system.
Tissue-resident macrophages were long considered to be recruited from bone-marrow progenitors that differentiated into mature cells upon seeding into tissues (1).
However, new evidence indicates that these macrophages are derived from yolk sac precursors, which arise during early development and persist locally via self-renewal (2).
In a similar vein, TAMs were once hypothesized to originate from circulating monocytes that were recruited in response to chemotactic signals released from tumor cells.
While monocyte-derived TAMs are continuously replenished by peripheral recruitment, a small proportion of TAMs can also arise from tissue-resident macrophages that are partially maintained through in situ proliferation (3, 4).
Circulating cells that are recruited into tissues and subsequently differentiate into TAMs include inflammatory monocytes and monocyte-related, myeloid-derived suppressor cells (MDSCs).
The differentiation of inflammatory Ly6CHigh monocytes into TAMs depends on RBPJ, the transcriptional regulator of Notch signaling (3).
Genetic ablation of the Rbpj gene reduced tumor burden in a spontaneous mouse model of breast cancer, indicating the absolute requirement of these monocyte-derived TAMs in supporting tumor growth (3).
A smaller subset of TAMs may also arise from Ly6CLow monocytes, which include cells that express the angiopoietin-2 (ANG2) receptor TIE2 (5).
These TIE2-expressing cells are recruited in response to the secretion of ANG2 by tumor endothelial cells and play non-redundant roles during tumor neovascularization (6).
By contrast, inhibition of STAT3 caused by upregulation of CD45 phosphatase activity is a key process that mediates the differentiation of MDSCs into mature TAMs (7).
MDSCs may exhibit a Ly6CHighLy6GNeg monocytic or a Ly6CIntLy6GHigh granulocytic endotype (8).
Since the monocytic MDSCs strongly resemble Ly6CHigh monocytes, it is hypothesized that these cells represent a precursor functional state of these inflammatory cells (8).
Tissue-resident macrophages coexist with recruited macrophages in tumors with potentially distinct roles.
In glioblastoma, TAMs are comprised of a mixed population of cells including resident microglia and bone marrow-derived monocytes and macrophages (9).
The relative contribution of these populations in glioma progression was investigated in a genetically engineered mouse model, in which the chemokine CX3CR1/CX3CL1 signaling was ablated in both microglia and inflammatory monocytes (9). CX3CR1 is expressed by circulating monocytes and exclusively by microglia in the central nervous system, while its ligand CX3CL1 is expressed by neurons and serves as a chemotactic signal.
Loss of Cx3cr1 in the host microenvironment facilitated the recruitment of Ly6CHigh “inflammatory monocytes” into tumor tissues, which were responsible for increased tumor incidence and shorter survival times in glioma-bearing mice.
By contrast, selective ablation of Cx3cr1 in microglia had no impact on glioma growth (9).
These results suggest that the tumor-promoting effect observed upon Cx3cr1ablation is conferred by infiltrating inflammatory monocytes and highlights the contrasting roles of bone marrow-derived and tissue resident-derived TAMs. However, since this may also depend on tumor type, the contribution of tissue-resident versus recruited TAMs in tumorigenesis warrants further investigation.
More information: Tumor-specific macrophage targeting through recognition of retinoid X receptor beta. Tang T, Wei Y, Kang J, She ZG, Kim D, Sailor MJ, Ruoslahti E, Pang HB. J Control Release. 2019 May 10;301:42-53
Journal information: Journal of Controlled Release
Provided by National Institutes of Health