Abstract
Activated B cells expressing the B7 family of molecules are efficient antigen presenting cells. However, most of B-lineage leukemia cells do not fullfill these criteria, having lost their immunostimulatory potential both in vivo and in vitro. We have established 3 human B-ALL(L3) cell-lines (BA-25, BA-78 and BA91) which in comparison with an established pre-B-ALL cell line (BA-127) and other patients-derived pre-B-ALL blast cells do express the costimulatory molecules CD80 and/or CD86, as well as other critical costimulatory molecules for T-cell stimulation such as: CD40, ICAM-I, LFA-3 and CD72. Flow cytometry analysis demonstrate that these B-ALL cells have an activated B-cell phenotype (CD19+, CD20+, CD22+, CD38+, CD71+, CD75+, DR+, HLA-class-I and HLA-class-II). Chromosomal translocations consistent with B-ALL (L3) were determined in BA-25 [t(2;28)(p12;q24)], BA-78 [t(8;14)(q24,q32)] and BA-91 [t(8;14) (q24,q32)]. No EBV DNA sequence was found in these B-ALL cell lines by Southern blot analysis with a probe for EBV genome. In mixted lymphocyte reaction (MLR) assay, Allogeneic T cells were stimulated by BA-25 and BA-91 cells at a B : T cells ratios of 1/30, 1/100, 1/300 and 1/1000. The BA-25 and BA-91 demonstrated 1-2 logs higher ccpm than the control pre-B-ALL cells (BA-127) which with no expression of B7 family molecules. The dose response curves for the B-cell mixture ratio display a linear pattern between 217-213 log2 ccpm. To determine T cell stimulation potential of the B-ALL cells, we co-cultured allogeneic PBMC with irradiated (3,000 Rad x-ray) BA-25 cells at 30:1 effector/stimulator ratio and achieved a 3–4 fold expansion of T cells after one round 7-day stimulation. Stimulation of allogeneic PBMC with BA-25 elicited proliferation of the T-cells and lysis of BA-25 as an target in a 4h Cr51 release assay, demonstrating BA-25 as an antigen presenting cell. Studies also show that the BA-25 cells could stimulate allo-cytolytic reactions by CTLs against other human leukemic target cells which lack these molecules and fail to stimulate T cell responses by themselves. Thus, BA-25 and other similar lines may prove useful for the provision of costimulatory signal needed to stimulate T-cell response against leukemias lacking these molecules.
Two separate functions of class II (Ia) molecules: T-cell stimulation and B-cell excitation.
