Summary CD45 (lymphocyte common antigen) is a receptor-linked protein tyrosine phosphatase that is expressed on all leucocytes, and which plays a crucial role in the function of these cells. On T cells the extracellular domain of CD45 is expressed in several different isoforms, and the particular isoform(s) expressed depends on the particular subpopulation of cell, their state of maturation, and whether or not they have previously been exposed to antigen. It has been established that the expression of CD45 is essential for the activation of T cells via the TCR, and that different CD45 isoforms display a different ability to support T cell activation. Although the tyrosine phosphatase activity of the intracellular region of CD45 has been shown to be crucial for supporting signal transduction from the TCR, the nature of the ligands for the different isoforms of CD45 have been elusive. Moreover, the precise mechanism by which potential ligands may regulate CD45 function is unclear. Interestingly, in T cells CD45 has been shown to associate with numerous molecules, both membrane associated and intracellular; these include components of the TCR-CD3 complex and CD4/CD8. In addition, CD45 is reported to associate with several intracellular protein tyrosine kinases including p56'^'^ and 59^^'"' of the src family, and ZAP-70 of the Syk family, and with numerous proteins of 29-34 kDa. These CD45-associated molecules may play an important role in regulating CD45 tyrosine phosphatase activity and function. However, although the role of some of the CD45-associated molecules (e.g. CD45-AP and LPAP) has become better understood in recent years, the role of others still remains obscure. This review aims to summarize recent findings on the role of CD45 and CD45-associated molecules in T cell activation, and to highlight issues that seem relevant to ongoing research in this area.

Purification of the complement component C1q from human serum using an established method resulted in the copurification of two 30 kDa proteins with an N-terminal sequence identical to human histidine-rich glycoprotein (HRG). Therefore, to explore the possibility that HRG can interact with C1q, we examined the ability of 81 kDa (native) and the 30 kDa proteins (presumably proteolytic N-terminal fragments of HRG) to bind to C1q, using both ELISA and optical biosensor techniques. Both forms of HRG were found to bind to the human complement component C1q and also to purified human and rabbit IgG by ELISA. Kinetic analyses of the HRG-C1q and HRG-IgG interactions using the IAsys biosensor indicate two distinct binding sites with affinities Kd1 0.78 x 10(-8) M and Kd2 3.73 x 10(-8) M for C1q, and one binding site with affinity Kd 8.5 x 10(-8) M for IgG. Moreover, the fact that both native and 30 kDa HRG bind to C1q and to IgG suggests that the IgG and C1q binding regions on HRG are located in the 30 kDa N-terminal region of the HRG molecule. The Fab region of IgG is likely to be involved in the HRG-IgG interaction since HRG also bound to F(ab')2 fragments with an affinity similar to that seen with the complete IgG molecule. Interestingly, the binding between HRG and IgG was significantly potentiated (Kd reduced from 85.0 to 18.9 nM) by the presence of physiological concentrations of Zn2+ (20 microM). Conversely, the presence of Zn2+ weakened the binding of HRG to C1q (Kd increased from 7.80 to 29.3 nM). Modulation of these interactions by other divalent metal cations was less effective with relative potencies being Zn2+ > Ni2+ > Cu2+. An examination of the effect of native and 30 kDa HRG on the formation of insoluble immune complexes (IIC) between ovalbumin and polyclonal rabbit anti-ovalbumin IgG revealed that physiological concentrations of HRG can markedly inhibit IIC formation in vitro. The results show that human HRG binds to C1q and to IgG in a Zn2+-modulated fashion, and that HRG can regulate the formation of IIC in vitro, thus indicating a new functional role for HRG in vivo.

A perfused liver system incorporating a Ca2+-sensitive electrode was used to study the long-term effects of glucagon and cyclic AMP on the mobilization of Ca2+ induced by phenylephrine, vasopressin and angiotensin. At 1.3 mM extracellular Ca2+ the co-administration of glucagon (10 nM) or cyclic AMP (0.2 mM) and a Ca2+-mobilizing hormone led to a synergistic potentiation of Ca2+ uptake by the liver, to a degree which was dependent on the order of hormone administration. A maximum net amount of Ca2+ influx, corresponding to approx. 3800 nmol/g of liver (the maximum rate of influx was 400 nmol/min per g of liver), was induced when cyclic AMP or glucagon was administered about 4 min before vasopressin and angiotensin. These changes are over an order of magnitude greater than those induced by Ca2+-mobilizing hormones alone [Altin & Bygrave (1985) Biochem. J. 232, 911-917]. For a maximal response the influx of Ca2+ was transient and was essentially complete after about 20 min. Removal of the hormones was followed by a gradual efflux of Ca2+ from the liver over a period of 30-50 min; thereafter, a similar response could be obtained by a second administration of hormones. Dose-response measurements indicate that the potentiation of Ca2+ influx by glucagon occurs even at low (physiological) concentrations of the hormone. By comparison with phenylephrine, the stimulation of Ca2+ influx by vasopressin and angiotensin is more sensitive to low concentrations of glucagon and cyclic AMP, and can be correlated with a 20-50-fold increase in the calcium content of mitochondria. The reversible uptake of such large quantities of Ca2+ implicates the mitochondria in long-term cellular Ca2+ regulation.

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