| Macrophages are ubiquitous innate immune cells that play prominent roles in host defense and disease. A central macrophage characteristic is the capacity to functionally "polarize" to distinct phenotypes in response to varying environmental stimuli. This heterogeneity is broadly divided into classic inflammatory responses (M1), including those mounted against infections, and alternative responses (M2), which promote immune suppression and wound healing. When macrophage "decision making" goes awry, dysfunctional macrophages promote disease. To build systems-level understanding of functional polarization, we investigated three aspects of macrophage decision making.;First, by subjecting macrophages to combinations of "incoherent" stimuli - those that individually promote either M1 or M2 phenotypes - we identified dominant regulatory modalities that reflect the quantitative behavior of the underlying networks. By characterizing macrophage responses to incoherent stimuli, we identified a central mechanism that regulates intracellular networks to initiate and maintain M2 phenotypes. We also identified mechanisms by which coupled multicellular networks may modulate the M1-M2 balance.;Second, by characterizing macrophage polarization at the single cell level, we demonstrated that under many conditions, individual macrophages within a single population polarize towards distinct M1-like and M2-like states. This apparently stochastic decision can be modulated by certain external stimuli, and we identified candidate regulators that may mediate this bias. Finally, we characterized the dynamic evolution of these ensembles of functional states and identified a role for intercellular communication in mediating the long-term resolution of population-level heterogeneity.;Third, we characterized the early stages of immune dysfunction during tumor progression. This work revealed the heterogeneous nature of the adaptive and innate immune response, both systemically and local to the tumor, at early stages of experimental melanoma. Such heterogeneity includes expression of contradictory signals, i.e., both inflammatory and anti-inflammatory cytokines, by adaptive and innate immune cells. We also identified the emergence of key early regulators of the tumor microenvironment, including myeloid derived suppressor cells, yolk-sac derived macrophages, and double negative T regulatory cells.;Together, these findings provide new insights into the function of local immune networks, inform immunotherapy design, and help to reconcile divergent reports of macrophage decision-making. |
