T-cell large granular lymphocyte (T-LGL)leukemia is a clonal lymphoproliferation of cytotoxic T cells (CTLs) associated with cytopenias. T-LGL proliferation seems to be triggered/sustained by antigenic drive; it is likely that hematopoietic progenitors are the targets in this process. The antigen-specific portion of the T-cell receptor (TCR), the variable beta (VB)-chain complementarity-determining region 3 (CDR3), can serve as a molecular signature (clonotype) of a T-cell clone. We hypothesized that clonal CTL proliferation develops not randomly but in the context of an autoimmune response. We identified the clonotypic sequence of T-LGL clones in 60 patients, including 56 with known T-LGL and 4 with unspecified neutropenia. Our method also allowed for the measurement of clonal frequencies; a decrease in or loss of the pathogenic clonotype and restoration of the TCR repertoire was found after hematologic remission. We identified 2 patients with identical immunodominant CDR3 sequence. Moreover, we found similarity between multiple immunodominant clonotypes and codominant as well as a nonexpanded, "supporting" clonotypes. The data suggest a nonrandom clonal selection in T-LGL, possibly driven by a common antigen. In contrast, the physiologic clonal CTL repertoire is highly diverse and we were not able to detect any significant clonal sharing in 26 healthy controls.

The 39-kDa receptor-associated protein (RAP) is a molecular chaperone for the low density lipoprotein receptor-related protein (LRP), a large endocytic receptor that binds multiple ligands. The primary function of RAP has been defined as promotion of the correct folding of LRP, and prevention of premature interaction of ligands with LRP within the early secretory pathway. Previous examination of the RAP sequence revealed an internal triplication. However, the functional implication of the triplicated repeats was unknown. In the current study using various RAP and LRP domain constructs, we found that the carboxyl-terminal repeat of RAP possesses high affinities to each of the three ligandbinding domains on LRP, whereas the amino-terminal and central repeats of RAP exhibit only low affinity to the second and the fourth ligand-binding domains of LRP, respectively. Using truncated soluble minireceptors of LRP, we identified five independent RAP-binding sites, two on each of the second and fourth, and one on the third ligand-binding domain of LRP. By coexpressing soluble LRP minireceptors and RAP repeat constructs, we found that only the carboxyl-terminal repeat of RAP was able to promote the folding and subsequent secretion of the soluble LRP minireceptors. However, when the ability of each RAP repeat to inhibit ligand interactions with LRP was examined, differential effects were observed for individual LRP ligands. Most striking, both the amino-terminal and central repeats, but not the carboxyl-terminal repeat, of RAP inhibited the interaction of ␣ 2 -macroglobulin with LRP. These differential functions of the RAP repeats suggest that the roles of RAP in the folding of LRP and in the prevention of premature interaction of ligand with the receptor are independent.The 39-kDa receptor-associated protein (RAP) 1 is an unique receptor antagonist. The target receptors for RAP are cysteinerich endocytic receptors that belong to the low density lipoprotein (LDL) receptor family (1). The four representative receptors in this family are the LDL receptor (2), the LDL receptorrelated protein (LRP, Ref.3), glycoprotein gp330/megalin (4), and the VLDL receptor (5). Among these receptors, LRP and gp330/megalin are large multifunctional receptors with multiple ligand-binding domains, which bind several structurally and functionally distinct ligands (for reviews, see Refs. 1 and 6). While RAP exhibits high affinities for LRP, gp330/megalin, and the VLDL receptor, it binds only weakly to the LDL receptor (7). Upon binding to these receptors, RAP inhibits the binding and/or endocytosis of all the ligands by the receptors. This unique feature of RAP has allowed its extensive use in biological studies of these endocytic receptors. Recent evidence has suggested that, under normal physiological conditions, RAP is an endoplasmic reticulum (ER) resident protein and functions within the early secretory pathway (8 -10). Using LRP as the target protein, it was found that RAP retained within the ER functions as a regulator of LRP activity by...