Identification and characterization of an active soluble form of human CD38 in normal and pathological fluids

Abstract

Human CD38 is a transmembrane glycoprotein involved in lymphocyte activation and adhesion to endothelium. The ectocellular domain of the molecule possesses properties of a bifunctional enzyme catalyzing both the synthesis from NAD+ and the hydrolysis of the calcium-releasing metabolite cyclic ADP-ribose (cADPR). Surface expression of CD38 (mCD38) is rapidly and almost completely down-modulated upon ligation by specific mAb in cells from different lineages. The data presented here also show that, in addition to the existence of a mCD38, a soluble form of CD38 (sCD38) is detectable in the cell culture supernatant of allo-activated T lymphocytes and of several tumor cell lines. sCD38 is also present in vivo and is assayable in normal (fetal serum and amniotic fluid) and pathological (serum and ascites from patients with multiple myeloma, and serum from patients with AIDS) biological fluids. Immunoaffinity chromatography, SDS-PAGE and Western blot analyses with mAb and polyclonal antibodies, along with metabolic labeling, yield a body of data concerning the structure of sCD38, which displays a M(r) of 39 kDa. Native sCD38 maintains the ability to inhibit the binding activity of different anti-CD38 mAb and still catalyzes the synthesis and the hydrolysis of cADPR at the same ratio observed with mCD38. Furthermore, cross-linking experiments indicate that the purified soluble molecule binds a 120 kDa molecule expressed by monocytoid cells and identified as a candidate ligand for human mCD38.

The metamorphosis of a molecule: from soluble enzyme to the leukocyte receptor CD38

“…Once there, CD38 can undergo shedding and internalization. A soluble form of human CD38 with a M r ϭ 39,000 was identified in the culture supernatants of mixed lymphocyte cultures and CD38 ϩ leukemic cell lines such as Raji (a Burkitt lymphoma) and Supt-1 (a T-acute lymphoblastic leukemia) [19] (Fig. 1).…”

“…For example, in 1998 it was reported that anti-CD38 mAb-reactive proteins in human serum are more abundant in cancer patients than in healthy controls [21]. Individuals with multiple myeloma can also have large amounts of soluble CD38 in their serum, ascites, and urine [19]. Amniotic fluid is also rich in soluble CD38 [19].…”

“…Individuals with multiple myeloma can also have large amounts of soluble CD38 in their serum, ascites, and urine [19]. Amniotic fluid is also rich in soluble CD38 [19].…”

“…For example, in the same individual, expression of CD38 on peripheral blood mononuclear cells (PBMC) varies with age, being highest in the first years of life [57]. Among adults, there are considerable inter-individual differences in CD38 expression on PBMC, in cytokine and proliferation response to anti-CD38 mAb triggering in PBMC [34], and in the amount of soluble CD38 in serum [19] but these differences may relate more to the stresses of the immune system than genetic programming.…”

Anti-CD38 Antibody Therapy: Windows of Opportunity Yielded by the Functional Characteristics of the Target Molecule

“…Instead, mature virgin and memory B lymphocytes express low levels of the molecule. CD38 also is found in soluble form in normal and pathological fluids (15) and in exosomes, which are membrane vesicles secreted by B cells and likely a component of an intercellular communication network (16). Clinical routine confirms that the cells of the myeloma clone do express CD38 in the overwhelming majority of patients, although at varying surface densities.…”