Lisa K. Gilliland

Tyrosine phosphorylation of CD19 in pre-B and mature B cells.

Cross‐linking of B cell surface immunoglobulins (sIg) results in activation of mature B cells and stimulates a molecular signaling mechanism for antigen‐specific B cell expansion and differentiation. This signaling pathway is dependent on tyrosine (Tyr) phosphorylation and results in the activation of sIg‐associated src family kinases and p72SYK. Rapid Tyr phosphorylation occurs on multiple protein substrates. Here we show that activation of B cells by cross‐linking sIg results in an increase in Tyr phosphorylation of the lineage‐restricted B cell surface antigen CD19, and show that it is a major substrate of activated Tyr kinase following sIg stimulation. Lower levels of constitutive CD19 Tyr phosphorylation occurred in most sIg+ mature B cell lines examined and in normal dense tonsillar B cells. We also find that when CD19 is Tyr‐phosphorylated it becomes competent to interact with SH2 domains suggesting a mechanism whereby, following B cell activation, CD19 could be linked to intracellular signaling pathways. In sIg‐ pre‐B cell lines, CD19 was expressed but was not constitutively phosphorylated on tyrosine. Upon CD19 cross‐linking, Tyr phosphorylation of CD19 was induced in sIg‐ pre‐B cell lines. CD19 cross‐linking also directly induced Tyr phosphorylation of CD19 and other substrates in mature B cells. The ability of CD19 to signal in the absence of sIg expression may provide important stimulation in pre‐B cell development.

Coengagement of CD2 with LFA-1 or VLA-4 by bispecific ligand fusion proteins primes T cells to respond more effectively to T cell receptor-dependent signals

To examine the effects of ligand engagement and accessory molecule juxtaposition on T cell receptor (TCR) signaling, we prepared LFA‐3/ICAM‐1 Rg and LFA‐3/VCAM‐1 Rg bispecific immunoglobulin fusion proteins (Rg, recombinant globulin). These novel fusion proteins allowed us to examine the effects of ligand driven co‐engagement of T cell proteins CD2 and LFA‐1 or CD2 and VLA‐4 on TCR‐dependent mobilization of intracellular Ca2+. We observed that preincubation of resting T cells with LFA‐3/ICAM‐1 Rg or LFA‐3/VCAM‐1 Rg fusion proteins resulted in significantly enhanced mobilization of intracellular Ca2+ following TCR‐accessory molecule cross‐linking relative to T cells preincubated with each of the monospecific Rgs alone or with combinations of the monospecific Rg fusion proteins. In addition, such coengagement stimulated TCR‐dependent activation and tyrosine phosphorylation of phospholipase Cγ1 (PLCγ1). These results suggest that when T cells interact with antigen presenting cells the engagement of multiple cell adhesion molecules such as CD2, LFA‐1, and VLA‐4 primes the T cell to respond more effectively to signals delivered through the TCR. J. Leukoc. Biol. 56: 444–452; 1994.

Molecular Order in Silk Secretions

Transmitted polarized light microscopy of various natural silk secretions reveals their ability to form nematic liquid crystalline phases. Observations of microstructure, together with a simple secondary structure analysis of known amino acid sequences in silk proteins, suggest that the rodlike structures forming the nematic phase are supramolecular aggregates, rather than individual rigid molecular segments. The optical birefringence of dragline fiber produced by controlled silking depends on the linear haul-off velocity, and can exceed the birefringence of naturally spun fibers; this suggests the possibility of in-vitro spinning of silk to obtain values of strength and stiffness even greater than those achieved in vivo.