Macrophage-activating factor

A macrophage-activating factor (MAF) is a lymphokine or other receptor based signal that primes macrophages towards cytotoxicity to tumors, cytokine secretion, or clearance of pathogens. Similar molecules may cause development of an inhibitory, regulatory phenotype. A MAF can also alter the ability of macrophages to present MHC I antigen, participate in Th responses, and/or affect other immune responses.[1][2]

Not to be confused with the transcription factor called MAF.

MAFs act typically in combination to produce a specific phenotype.[2]

Macrophage activated phenotypes

Macrophages inherently display tissue and environment-dependent plasticity.[3] In addition, the phenotypes of the macrophages in a certain environment play a fundamental role in determining the immune activity and response within the tissue.

Depending on the combination of MAFs signaling to the macrophage, the macrophage’s activated phenotype becomes one of three major categories: classically activated, wound healing, or regulatory. Regulatory-phenotype macrophages have only recently been recognized as an important contributor to tissue microenvironments.

Tumor-associated macrophages may be any of these types, and they have been found to be important players in the tumor microenvironment. Analysis of the macrophage population and signaling in a tumor may provide useful clinical data.[2]

Clarifications on terminology
  • Macrophages have been classified as M1 or M2 depending on the adaptive immune response that elicited the phenotype: Th1 or Th2 respectively.[2][4][5]
  • The phrase 'alternatively activated macrophage' is used to refer to M2 macrophages.[2]
  • Regulatory macrophages do not fit into the M1/M2 classification system, and they display different markers.[6]

Classically activated macrophages

After receiving signaling from both IFNγ and TNF, macrophages acquire a phenotype with higher activity against both pathogens and tumor cells. They also secrete inflammatory cytokines. IFNγ signaling can initially originate from Natural Killer (NK) cells, but adaptive immune cells are required to sustain a population of classically activated macrophages.

Toll-like receptor agonists may also cause macrophage activation.[2]

Wound healing macrophages

Interleukin 4, secreted by granulocytes after tissue damage or by adaptive immune cells within a Th2 response, causes macrophages to secrete minimal amounts of pro-inflammatory cytokines and to have lower activity against intracellular pathogens. They also promote extracellular matrix synthesis via production of ornithine, via arginase; this is used as a precursor for extracellular matrix components. The overall result is a macrophage population that promotes wound healing.[2]

The specific roles macrophages play in the Th2 response are still under investigation.[2]

Regulatory macrophages

Glucocorticoids can contribute to the development of regulatory macrophages. These macrophages produce Interleukin 10 and inhibit immune system response (See below for Effect on cancer). Tumor-associated macrophages may contain a large population of regulatory macrophages.[2]

Effect on cancer

Initially, MAFs were thought to increase a macrophage’s cytotoxic response, allowing enhanced clearance of the tumor cells. However, they also have wider ranging effects. Chronic inflammation associated with activated macrophages may lead to the development of neoplasia, such as those found surrounding tuberculosis scars.

Dysregulation of macrophage activation may cause increased inflammation and eventual neoplasia.[2]

Moreover, macrophages infiltrating the tumor microenvironment can transition towards a regulatory phenotype. Regulatory macrophages produce Interleukin 10, which can inhibit cytotoxic responses of other lymphocytes to cancer cell antigens. The stromal reaction surrounding a tumor, as well as prostaglandins and hypoxia may play a role in this transition.[2]

Epithelial-mesenchymal transition has been found to be influenced by all types of macrophages, which cause both pro and anti-inflammatory responses that can promote EMT.[7]

Non-cytokine examples of macrophage-activating factors

Pathogenic antigens can bind to toll-like receptors that stimulate macrophage activation and response. Examples include heat shock proteins released during apoptosis, and bacterial lipopolysaccharide.[2]

Examples

Miscellaneous

It has been suggested that MAF can be formed by probiotic bacteria in a yoghurt medium. This probiotic mixture has been found to be helpful in various immune disturbances including ME/CFS.[1]

References
  1. Mosser DM (February 2003). "The many faces of macrophage activation". J. Leukoc. Biol. 73 (2): 209–12. doi:10.1189/jlb.0602325. PMID 12554797.
  2. Mosser DM, Edwards JP (December 2008). "Exploring the full spectrum of macrophage activation". Nat. Rev. Immunol. 8 (12): 958–69. doi:10.1038/nri2448. PMC 2724991. PMID 19029990.
  3. Giorgio S (September 2013). "Macrophages: plastic solutions to environmental heterogeneity". Inflamm. Res. 62 (9): 835–43. doi:10.1007/s00011-013-0647-7. PMID 23872927.
  4. Mills CD, Kincaid K, Alt JM, Heilman MJ, Hill AM (2000). "M-1/M-2 macrophages and the Th1/Th2 paradigm". J Immunol. 164 (12): 6166–73. doi:10.4049/jimmunol.164.12.6166. PMID 10843666.
  5. Weisser SB, et al. (2013). Generation and characterization of murine alternatively activated macrophages. Methods Mol. Biol. Methods in Molecular Biology. 946. pp. 225–39. doi:10.1007/978-1-62703-128-8_14. ISBN 978-1-62703-127-1. PMID 23179835.
  6. Yu WG, et al. (2013). "IFN-γ-induced iNOS Expression in Mouse Regulatory Macrophages Prolongs Allograft Survival in Fully Immunocompetent Recipients". Mol. Ther. 21 (2): 409–422. doi:10.1038/mt.2012.168. PMC 3594012. PMID 22929659.
  7. Helm O, et al. (Jan 23, 2013). "Tumor-associated macrophages exhibit pro- and anti-inflammatory properties by which they impact on pancreatic tumorigenesis". Int. J. Cancer. 135 (4): 843–61. doi:10.1002/ijc.28736. PMID 24458546.
  8. DeFilippis RA, et al. (July 2012). "CD36 repression activates a multicellular stromal program shared by high mammographic density and tumor tissues". Cancer Discov. 2 (9): 826–39. doi:10.1158/2159-8290.CD-12-0107. PMC 3457705. PMID 22777768.
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