CCR2 deficiency compromises antibody response to T cell-dependent antigens
To address the impact of CCR2 deletion on B cell function, we examined the T cell-dependent antigen immune response in WT and CCR2 KO mice. Mice were first injected i.p. with 40 μg NP-KLH and boosted at 14 days post-prime, then euthanized 5 days later. Splenic B cells were harvested to determine the peripheral B cell subsets and antibody-secreting cells (ASCs) (Fig. S1J ). In contrast to the un-immunized CCR2 KO mice, immunized KO mice exhibited a notable increase in FO, MZ, T1, and T2 B cell number, albeit with no significant difference in their proportion to the whole splenic B cells, except for T2 B cells (Fig. 7A-F ). Furthermore, there were no changes in GCB cells before and after immunization (Fig. 7G-H ). Moreover, the percentage or the absolute cell number of ASCs, including plasma cells (PC) and plasmablast cells (PBC), were dramatically reduced in CCR2 KO mice (Fig. 7I-K ). Additionally, in CCR2 KO mice, there was a trend toward a decrease of the percentage and cell number in memory B cells (MBC) (Fig. 7L-M ).
NP-specific IgM and IgG levels in serum from primed and boosted mice were measured using ELISA. We observed that the NP-specific IgM levels of CCR2 KO mice were significantly reduced compared to those of WT mice (Fig. 7N-O ), particularly after the initial immunization. Paradoxically, we noticed an increased number of lymphatic follicles that contained anatomical GCs in the spleen of immunized CCR2 KO mice, as well as enlarged structures (Fig. 7P ). The enlarged lymphatic follicles but decreased ASCs in CCR2 deficient mice might indicate an impairment in the differentiation from FO B cells to ASCs; however, future research is necessary to confirm this hypothesis. Collectively, CCR2 plays an important role in the immune response of B cells, and its absence was sufficient to lead to a compromised immune response.