The role of CD45RO in antithymocyte globulin's stimulation of primitive haemopoietic cells

Summary. We have found that antithymocyte globulin (ATG), an equine antibody with proven efficacy in aplastic anaemia (AA), has a direct stimulatory effect on primitive haemopoietic cells from normal donors. This growth stimulation may be mediated via anti-CD45RO activity present in the ATG preparation. Addition of unabsorbed ATG enhanced colony growth at 21 d in the blast colony forming cell (Bl-CFC) assay. Prior absorption of ATG by incubation with the CD45RO⁺ MOLT-4 cell line resulted in the loss of enhancement. Absorption by MOLT-4 cells preincubated with anti-CD45RO mAb, UCHL-1, restored ATG's stimulatory effect. The Bl-CFC could also be stimulated to grow by the addition of UCHL-1 directly. Incubation of the primitive haemopoietic cells for 4 h with ATG was associated with a decline in the antigenic density of CD45RO, a tyrosine phosphatase. This down-regulation may upset the balance between growth factor-induced tyrosine kinase activation and tyrosine phosphate dephosphorylation resulting in increased growth of primitive cells. a possible factor in the sustained recovery of haemopoiesis seen in AA patients after ATG treatment.