CCR2 is expressed in immature B cells and is down-regulated as part of the transition between immature and mature B cells. (A-D) RT-PCR of (A) immature (IgD^– B220⁺) cells derived from control or invariant chain-deficient (Ii^–/–) mice; control mature (IgD⁺ B220⁺) cells and 70Z/3 cells were purified. Total RNA was isolated, and reverse transcription was carried out by using Superscript II RT. (B-C) B cells (B220⁺) from control (B) or invariant chain deficient (Ii^–/–) (C) mice were separated into IgD⁺ and IgD^– cells; IgD^– cells were separated according to their CD21 expression using the MACS system. Total RNA was isolated, and reverse transcription was carried out by using Superscript II RT. The results presented are representatives of 5 different experiments. (D) T1, T2, and mature (M) B cells were separated by fluorescence activated cell sorting (FACS). Total RNA was isolated, and reverse transcription was carried out by using Superscript II RT. (E) FACS analysis of CCR2 expression on 70Z/3 cells. 70Z/3 cells were stained with anti-CCR2 or a control antibody. The results presented are representative of 3 different experiments. Brackets indicate percentage of CCR2-positive cells.