Abstract In addition to antimicrobial activity, macrophages regulate tissue development, remodeling, and repair. In terms of neoplastic growth, tumor-associated macrophages (TAMs) are even considered pro-tumorigenic based on their angiogenic and T-cell suppressive properties. In multiple myeloma (MM) macrophages represent an abundant component of the stromal cell compartment and are believed to support proliferation, survival, and drug resistance of MM cells. Those pro-tumorigenic functions are partly instructed by T-helper (TH)-2 cytokines such as interleukin-4 (IL-4) and IL-10, which skew TAM differentiation towards an alternatively activated, or so-called M2-like, phenotype. In contrast, M1 macrophages generally considered as potent effectors in response to microbial products or interferon-γ (IFN-γ), are characterized by superior antigen-presentation, abundant production of pro-inflammatory cytokines such as interleukin-12 (IL-12), and consequently, promote a polarized type I immune response against infections as well as malignant cells. Transcriptional regulation is a key determinant for macrophage polarization. The Ikaros (IKZF1) transcription factor is critical for lymphoid development and is found in all hematopoietic progenitors as well as in T-, B-, NK-cells and macrophages. Interestingly, IKZF1 is overexpressed in MM and selectively degraded by lenalidomide, which is approved in MM therapy. The potential role of IKZF1 in modulating macrophage polarization has not been elucidated yet. Here, we show that IKZF1expression is found highly elevated in M2-like macrophages and in MM TAMs. IKZF1 deletion in human macrophages by small interfering RNA (siRNA) or by lenalidomide yields an upregulation of M1-specific cytokines (IL-12 and IL-1b), chemokines (CXCL10 and CCL5), and costimulatory molecules (CD86 and CD40) and leads to a potent TH1-TH17 response. In fact, lenalidomide-pretreated macrophages display strong tumoricidal effects when co-cultured with MM cell lines as opposed to their untreated counterparts promoting MM proliferation and viability. Utilizing immunoprecipitation-sequencing (ChIP-Seq) we reveal that IKZF1 governs IRF4 and IRF5 expression in human macrophages. Recent studies demonstrate that IRF4 controls M2 macrophage polarization, while IRF5 regulates the M1 phenotype respectively. We could show that a lenalidomid-mediated M1-phenotype induction is efficiently abrogated by IRF4 overexpression or IRF5 silencing. Overall, these findings unravel a novel role for IKZF1 in orchestrating macrophage polarization via the IRF4/IRF5 pathway. Disclosures No relevant conflicts of interest to declare.