Table 2.

CD19+ B cells and CD138+ plasma cells in MM PBMC resist treatment with rituximab (anti-CD20).

Timepoint (n)	% of PBMC
	Total CD19+ B cells	Total CD138+ PC	CD138+20+ PC
Pre-treatment (6)	19.9 ± 4.5	0.4 ± 0.15	0.26 ± 0.14
1–2 months (10) after initial treatment	17.3 ± 2.6
3–5 months (9) after initial treatment	29.3 ± 6.8
3–5 months (8) after initial treatment		0.38 ± 0.09	0.23 ± 0.15

Values represent the mean % ± SE for the indicated subset of PBMC.

n is the number of blood samples analyzed from the 6 patients receiving Rituximab treatment. The values include 1–2 samples per patient, averaged for each patient having more than one sample before inclusion in the table, taken within the time window indicated. Absolute numbers are provided in Table 4. PC = plasma cells.

PBMC taken from patients that had been treated with rituximab at the indicated time before or after treatment were stained with CD19 (FMC63), CD20 and CD138 or isotype matched control antibodies. The percent of CD19+ B cells indicted in the table is the aggregate number, including both includes both monocytoid and lymphocyte scatter gates. Staining with anti-CD19 was used to ensure detection of all B cells including any that had bound rituximab thereby obscuring their CD20 epitopes. Overall, the % of CD19+ B cells was comparable to the % of CD20+ B cells (as shown in Table 3) indicating that the extent of surface rituximab binding was likely to be minor. Plasma cells were identified as CD138+ cells with or without concomitant CD20 staining.