Circulating monocytes are divided into two major, phenotypically and functionally distinct subsets: Gr-1(hi) "inflammatory" and Gr-1(lo) "resting" monocytes. One of the unique cellular abnormalities in lupus-prone mice is monocytosis, which is characterized by a selective expansion of Gr-1(lo) monocytes and dependent on the expression of stimulatory IgG Fc receptors (FcγR). We speculated that IgG immune complexes containing nuclear antigens could stimulate Gr-1(hi) monocytes through interaction with FcγRs and then TLR7 and TLR9, thereby promoting the maturation towards Gr-1(lo) monocytes. In the present study, we assessed this hypothesis by analyzing effects of TLR9 or TLR7 agonist on monocytes in vivo. The analysis of various surface markers differentially expressed on both subsets of monocytes in combination with selective depletion of either subset revealed that within 48 h after injection of the TLR9 agonist CpG, approximately one third of Gr-1(hi) monocytes became phenotypically identical to Gr-1(lo) monocytes. In addition, we observed approximately two-fold increases in the total monocyte population 8-24 h after injection of CpG. Moreover, the activation of TLR9 resulted in an increased expression of stimulatory FcγRIV relative to inhibitory FcγRIIB on monocytes, thereby enhancing their responsiveness to IgG immune complexes. Essentially identical results were obtained after stimulation of TLR7 with a synthetic agonist (1V136). Our results indicate that the activation of TLR7 and TLR9 not only induced the maturation of a fraction of Gr-1(hi) monocytes towards Gr-1(lo) monocytes but also promoted the overall generation of monocytes, thereby supporting the critical role of TLR7 and TLR9 for the development of monocytosis in lupus-prone mice.