Which factors stimulate the M1 macrophage phenotype?

The M1 macrophage phenotype is primarily stimulated by microbial products and pro-inflammatory cytokines. This phenotype is known for its role in the immune response, particularly in fighting infections and tumors. When macrophages encounter microbial components, such as lipopolysaccharides (LPS) from bacteria, they are prompted to express a variety of pro-inflammatory mediators. Similarly, pro-inflammatory cytokines, such as interferon-gamma (IFN-γ) and tumor necrosis factor-alpha (TNF-α), further enhance the activation of M1 macrophages, leading to increased production of inflammatory cytokines, reactive oxygen species, and enhanced phagocytic activity.

M1 macrophages are characterized by their ability to induce Th1 responses and activate T cell responses, thus playing a crucial role in the body's defense mechanisms. This contrasts with the other options, which either represent factors that do not typically stimulate M1 polarization or pertain to different immune responses that do not directly promote the M1 phenotype.