The stimuli inducing expression of single immunoglobulin IL-1-related receptor (SIGIRR) and the relevant regulatory mechanisms are not well defined. Transforming growth factor ß1 (TGFß1) delays internalization of neurokinin-1 receptor (NK1R) and subsequently enhances cellular signaling. This study investigated the effect of TGFß1 on SIGIRR protein production by human M1 macrophages in response to stimulation with substance P (SP). SP caused upregulation of SIGIRR expression in a concentration-dependent manner, whereas aprepitant (an NK1R inhibitor) blunted this response. Silencing p38γMAPK or TAK-1 partially attenuated the response to SP stimulation, while TGFß1/2/3 siRNA dramatically diminished it. SP induced much greater SIGIRR protein production than either lipopolysaccharide (a TLR4 agonist) or resiquimod (a TLR7/8 agonist). Unexpectedly, silencing of transcription factor specificity protein 1 (Sp1) led to significant upregulation of SIGIRR expression after SP stimulation, while KLF2 siRNA only partially enhanced it and Fli-1 siRNA reduced it. SP also upregulated TGFß1 expression, along with a corresponding increase of SIGIRR protein, whereas silencing TGFß1/2/3 blunted these responses. Sp1 siRNA or mithramycin (a gene-selective Sp1 inhibitor) significantly enhanced the expression of TGFß1 and SIGIRR by macrophages after SP stimulation. Importantly, this effect of Sp1 siRNA on TGFß1 and SIGIRR was blunted by siRNA for Smad2, Smad3, or Smad4, but not by TAK-1 siRNA. Next, we investigated the influence of transcription factor cross-talk on SIGIRR expression in response to SP. Co-transfection of macrophages with Sp1 siRNA and C/EBPß or TIF1ß siRNA attenuated the upregulation of SIGIRR by SP, while a combination of Sp1 siRNA and Fli-1 siRNA dramatically diminished it. In conclusion, TGFß1 may be an intermediary between SP/NK1R activation and SIGIRR expression in Sp1 siRNA-transfected macrophages. In addition, Sp1 modulates TGFß1/Smad signaling and negatively regulates SIGIRR protein production by macrophages after SP stimulation.