The high affinity receptor for immunoglobulin G (IgG), FcλRI, is constituitively expressed on cells of monocyte-macrophage lineage. It plays a central role in orchestrating both humoral and cellular arms of the immune system, mediating a number of important effector functions including phagocytosis and antibody-directed cellular cytotoxicity (Da‘ron, 1997). Although the cytoplasmic tail of FcλRI contains no known signalling motifs, receptor aggregation appears to cause recruitment and activation of soluble tyrosine kinases by interaction with accessory signalling molecules (ASMs) within the plasma membrane. Two candidate ASMs have been proposed: the λ chain, originally described as part of the IgE receptor (FcπRI) complex, and the low-affinity IgG receptor, FcλRII.

We have previously shown that aggregation of FcλRI, in the human monocytic cell line U937, leads to activation of very different intracellular signalling pathways depending on which ASM is involved (Melendez et al. 1998). In U937 cells pretreated with dibutyryl cAMP (dbcAMP), FcλRI signalling through FcλRII leads to activation of phospholipase Cλ1 generating inositol 1,4,5 trisphosphate (IP₃) and oscillations in intracellular Ca²⁺ concentration ([Ca²⁺]_i). In contrast, λ chain-mediated FcλRI signalling (as seen in interferon λ-treated U937 cells) leads to phospholipase D activation and generation of a single discrete rise in [Ca²⁺]_i by means of a sphingolipid second messenger. By inhibiting surface expression of either the λ chain or FcλRII by pretreatment with antisense oligonucleotides, we have examined the role of both ASMs in FcλRI-triggered phosphoinositol and Ca²⁺ signalling in dbcAMP-treated U937 cells.

Compared with controls, we found that inhibition of λ chain expression resulted in an increased rate of total inositol phosphate generation, increased [IP₃]_i levels and an increased proportion of cells demonstrating FcλRI-triggered Ca²⁺ oscillations. In contrast, inhibition of FcλRII expression led to complete inhibition of FcλRI-triggered [IP₃]_i rise and a reduced proportion of cells undergoing Ca²⁺ oscillations. These results suggest an absolute requirement for FcλRII in FcλRI-triggered IP₃ generation and tonic inhibition of this signalling pathway by the λ chain.

In addition, measurement at time points shortly after FcλRI aggregation of dbcAMP-treated cells revealed considerable fluctuations in [IP₃]_i. The temporal variations observed in [IP₃]_i correlate well with summated [Ca²⁺]_i measurements of 30 cells undergoing FcλRI-triggered Ca²⁺ oscillations. Furthermore, these fluctuations in [IP₃]_i reflect inverse changes in levels of intracellular inositol 1,3,4,5-tetraphosphate and both can be inhibited by buffering rises in [Ca²⁺]_i by pre-incubation of cells with BAPTA-AM ester (10 µM for 30 min). These results can, at least partly, be explained by intermittent Ca²⁺-dependent activation of IP₃ kinase leading to fluctuations in [IP₃]_i and subsequently [Ca²⁺]_i and provide insight into possible mechanisms involved in generating FcλRI-mediated Ca²⁺ oscillations.