The post-GC humoral immune response is compromised in the absence of CD138 expression. (A-C) Mice lacking CD138 expression (sdc1^−/−) or WT mice were immunized subcutaneously with NP(16)-OVA precipitated in alum and bled on the indicated days. (A) NP-specific levels of total IgG were measured by ELISA. Quantification represents μg/mL IgG. Dotted line highlighted by # represents the mean NP-specific IgG response of unimmunized naïve mice. (B) NP-specific levels of total IgM were measured by ELISA. Quantification represents O.D. Dotted line highlighted by # represents the mean NP-specific IgM response of unimmunized naïve mice. (C) ELISA was used to examine affinity maturation by measuring the ratio from the binding of low-affinity IgG to NP(15) and higher-affinity IgG binding to NP(3). (D) A representative ELISPOT image of draining LN cells from WT or sdc1^−/− mice 8 days after immunization. Graph shows quantification (mean ± SEM) of NP-specific IgG ASCs per 10⁶ LN cells. (E) Graph depicts the total number of IgM⁺ or IgG⁺ antigen-specific (NP⁺YFP⁺) ASCs in the LN of immunized WT and sdc1^−/− mice (left). Stacked bar graph shows the percentage of IgM⁺ or IgG⁺ antigen-specific (NP⁺) YFP⁺ ASCs on day 8 following immunization of WT and sdc1^−/− mice (right). (F) ELISPOT analysis of BM cells from WT and sdc1^−/− mice 60 days after immunization. Graph represents mean (± SEM) of the number of NP-specific ASCs per 10⁶ BM cells. (G-H) CD4⁺B220⁻PD-1⁺CXCR5⁺Foxp3⁻ TFH cells and CD4⁺B220⁻PD-1⁺CXCR5⁺Foxp3⁺ TFR cells were examined by flow cytometry on day 7 following immunization of WT or sdc1^−/− mice. Gating strategy and representative dot plots of TFH and TFR cells are shown. (G). Graphs show the percentage of TFH or TFR cells in the LN of non-immunized (naïve) and immunized mice. (I) B220⁺GL7⁺Fas⁺ GC B cells were measured by flow cytometry on day 7 following immunization. Graph represents the percentage of GC B cells in either naïve or immunized WT or sdc1^−/− mice. *P < .05; **P < .01; ***P < .001. ns, not significant; O.D., optical density; TFH, T follicular helper; TFR, T follicular regulatory.