2. Analysis of anti-FVIII production by ELISpot reveals that the first and third well displayed contained FVIII-specific ASCs. Six spots are observed in the first well whereas 46 spots are present in the third well. This indicates that FVIII-specific ASCs increased in number following stimulation selleck inhibitor by CD40L and appropriate cytokines. In the other wells, no spots corresponding to ASCs producing anti-FVIII IgG were observed. The presence of FVIII-specific ASCs was also detected using ELISA (Fig. 2b). Seven of 55 wells analysed were strongly positive whereas low levels of anti-FVIII antibodies were observed in two wells. The two wells

that were positive in the ELISpot were also strongly positive when ELISA was used as a read-out for anti-FVIII antibodies (Fig. 2; boxed wells in ELISA correspond BVD-523 research buy to the six wells for which results of the ELISpot are displayed). Overall, detection of anti-FVIII antibodies by ELISA appeared more sensitive when compared with ELISpot. The prolonged incubation of B cells for 9–10 days may favour the detection of anti-FVIII antibodies using ELISA as a result of accumulation of IgG in the conditioned medium. Analysis by ELISpot requires the presence of viable IgG-producing cells which is optimally assessed 6 days after stimulation [35]. Based on the number of positive wells the frequency of FVIII-specific memory B

cells can be calculated assuming that a positive well corresponds to the presence of a single FVIII-specific memory B cell. For the patient sample analysed in Fig. 2a, seven (ELISpot) or nine (ELISA) of 55 wells contained FVIII-specific memory B cells. The frequency of FVIII-specific B cells thus ranges from 13 to 16 cells per 105 CD19+ B cells for this medchemexpress patient. Analysis of peripheral blood samples from additional haemophilia A patients with inhibitors revealed that FVIII-specific memory B cells could be detected in four of five patients analysed [33]. The frequency of FVIII-specific

memory B cells in these patients ranged from 5 to 24 per 105 CD19+ B cells as determined using ELISA as a read-out (Table 1). In parallel, we determined the percentage of FVIII-specific memory B cells by comparing the number of IgG and anti-FVIII IgG-producing ASCs (Table 1). This analysis revealed that 0.07–0.35% of circulating memory B cells can develop into FVIII-specific ASCs [33]. These values are similar to the frequency of 0.24% as reported in a patient with an inhibitor titre of 1000 BU mL−1 [34] and are in the same range as observed for antigen-specific memory B cells in peripheral blood of HIV-infected patients and in normal individuals following smallpox, hepatitis B and tetanus toxoid [35,41–43]. In one patient the frequency of FVIII-specific memory B cells was below the limit of detection (frequency <1 per 105 B cells).