Recent studies have described the development of distinct functional subsets of macrophages in association with cancer, autoimmune disease, and chronic infections. Based on the ability of Th1 vs Th2 cytokines to promote opposing activities in macrophages, it has been proposed that macrophages develop into either type 1 inflammatory or type 2 anti-inflammatory subsets. As an alternative to the concept of subset development, we propose that macrophages, in response to changes in their tissue environment, can reversibly and progressively change the pattern of functions that they express. As demonstrated herein, macrophages can reversibly shift their functional phenotype through a multitude of patterns in response to changes in cytokine environment. Macrophages display distinct functional patterns after treatment with IFN-γ, IL-12, IL-4, or IL-10 and additional functional patterns are displayed depending on whether the cytokine is present alone or with other cytokines and whether the cytokines are added before or concomitantly with the activating stimulus (LPS). Sequential treatment of macrophages with multiple cytokines results in a progression through multiple functional phenotypes. This ability to adapt to changing cytokine environments has significant in vivo relevance, as evidenced by the demonstration that macrophage functional phenotypes established in vivo in aged or tumor-bearing mice can be altered by changing their microenvironment. A concept of functional adaptivity is proposed that has important implications for therapeutic targeting of macrophages in chronic diseases that result in the dominance of particular functional phenotypes of macrophages that play a significant role in disease pathology.

Herein, we demonstrate a role of AMP-activated protein kinase (AMPK) as a potent counterregulator of inflammatory signaling pathways in macrophages. Stimulation of macrophages with anti-inflammatory cytokines (i.e., IL-10 and TGFβ) resulted in the rapid phosphorylation/activation of AMPK, whereas stimulation of macrophages with a proinflammatory stimulus (LPS) resulted in AMPK dephosphorylation/inactivation. Inhibition of AMPKα expression by RNA interference dramatically increased the mRNA levels of LPS-induced TNF-α, IL-6, and cyclooxygenase-2. Likewise, expression of a dominant negative AMPKα1 in macrophages enhanced TNF-α and IL-6 protein synthesis in response to LPS stimulation, while diminishing the production of IL-10. In contrast, transfection of macrophages with a constitutively active form of AMPKα1 resulted in decreased LPS-induced TNF-α and IL-6 production, and heightened production of IL-10. In addition, we found that AMPK negatively regulated LPS-induced IκB-α degradation and positively regulated Akt activation, accompanied by inhibition of glycogen synthase kinase β and activation of CREB. Thus, AMPK directs signaling pathways in macrophages in a manner that suppresses proinflammatory responses and promotes macrophage polarization to an anti-inflammatory functional phenotype.

There has been substantial research activity in the past decade directed at phenotyping macrophage lineages and defining macrophage functional subsets or patterns of activity. The emphasis over the past 2-3 years has been to divide macrophage functional patterns into type 1 (Th1-driven) or type 2 (Th2-driven) functions. However, a huge array of environmental factors (including cytokines, chemokines, pattern recognition receptors, hormones) differentially regulates macrophage response patterns, resulting in the display of numerous distinct, functional phenotypes. Upon stimulation, a macrophage does not display just a single set of functions but rather displays a progression of functional changes in response to the progressive changes in its microenvironment. The remarkable ability of monocytes and tissue macrophages to adapt to changes in their microenvironment challenges the thesis that macrophages displaying unique tissue-specific or response-specific, functional patterns represent distinct lineages. With the exception of mature osteoclasts and mature dendritic cells, evidence supporting stable differentiation as the basis for macrophage functional heterogeneity is equivocal. The concept of whether macrophages develop into functional subsets as opposed to continuously adapting their functional pattern in response to the changing environment of a progressive inflammatory response is important to resolve from the perspectives of therapeutic targeting and understanding the role of macrophages in disease pathogenesis.

Neuroglial cells define brain homeostasis and mount defense against pathological insults. Astroglia regulate neurogenesis and development of brain circuits. In the adult brain, astrocytes enter into intimate dynamic relationship with neurons, especially at synaptic sites where they functionally form the tripartite synapse. At these sites astrocytes regulate ion and neurotransmitter homeostasis, metabolically support neurons and monitor synaptic activity; one of the readouts of the latter manifests in astrocytic intracellular Ca2+ signals. This form of astrocytic excitability can lead to release of chemical transmitters via Ca2+-dependent exocytosis. Once in the extracellular space, gliotransmitters can modulate synaptic plasticity and cause changes in behavior. Besides these physiological tasks, astrocytes are fundamental for progression and outcome of neurological diseases. In Alzheimer’s disease, for example, astrocytes may contribute to the etiology of this disorder. Highly lethal glial-derived tumors use signaling trickery to coerce normal brain cells to assist tumor invasiveness. This review sheds new light on the brain operation in health and disease, but also points to many unknowns.

Tumor-associated macrophages (TAMs) play a major role in promoting tumor growth and metastasis and in suppressing the antitumor immune response. Despite the immunosuppressive environment created by the tumor and enforced by tumor-associated macrophages, treatment of tumor-bearing mice with IL-12 induces tumor regression associated with appearance of activated NK cells and activated tumor-specific CTLs. We therefore tested the hypothesis that IL-12 treatment could alter the function of these tumor-associated suppressor macrophages. Analysis of tumor-infiltrating macrophages and distal TAMs revealed that IL-12, both in vivo and in vitro, induced a rapid (<90 min) reduction of tumor supportive macrophage activities (IL-10, MCP-1, migration inhibitory factor, and TGFβ production) and a concomitant increase in proinflammatory and proimmunogenic activities (TNF-α, IL-15, and IL-18 production). Similar shifts in functional phenotype were induced by IL-12 in tumor-infiltrating macrophages isolated from the primary tumor mass and in TAMs isolated from lung containing metastases, spleen, and peritoneal cavity. Therefore, although TAMs display a strongly polarized immunosuppressive functional profile, they retain the ability to change their functional profile to proinflammatory activities given the appropriate stimulus. The ability of IL-12 to initiate this functional conversion may contribute to early amplification of the subsequent destructive antitumor immune response.