ACTIVATION AND INTERNALIZATION OF P56(LCK) UPON CD45 TRIGGERING OF JURKAT CELLS

Relationships between CD45 and p56(lck) have been suggested by co-immu noprecipitation of both proteins and by dephosphorylation of the p56(l ck) regulatory site, Tyr 505, by CD35 in vitro. We investigated whethe r the kinase activity of p56(lck) is modulated in T cells triggered vi a CD45. We showed that incubation of Jurkat cells with a combination o f two anti-CD45 monoclonal antibodies (mAb) (MC5/2 + D3/9) induced an increase in p56(lck) kinase activity, while a single mAb did not. Unde r these conditions, p56(lck) underwent two consecutive waves of activa tion. This was accompanied by internalization of the kinase and by a t ime-dependent increased accessibility of CD45 phosphatase at the plasm a membrane. Similarly, activation and internalization of p56(lck) were observed using a combination of anti-CD45 (MC5/2) and anti-CD2 (T11(2 )) mAb, suggesting that a functional complex consisting of CD45, CD2 a nd p56(lck) was formed upon cell triggering. Taken together, these res ults suggests that: (i) CD45 participates in the regulation of p56(lck ) kinase activity in vivo and that (ii) CD45 could play a mediator rol e in the stimulation and endocytosis of p56(lck) through the CD2 pathw ay.