The human LT system. III. Characterization of a high molecular weight LT class (complex) composed of the various smaller MW LT classes and subclasses in association with Ig-like molecules

Lymphotoxin: cloning, regulation and mechanism of killing

The gene for murine lymphotoxin (MuLT) has been cloned from a cDNA library prepared using poly(A)+ RNA from an activated murine IL-2-maintained cloned T cell line (21C11). This was accomplished with a MuLT BamHI fragment isolated from a murine genomic library by hybridization to a human LT cDNA probe. Northern blot analysis with RNA from 21C11, an L3T4+ (CD4+-equivalent) ovalbumin-specific class II-restricted T cell line, revealed a 15S band that hybridized to this MuLT fragment. A cDNA library prepared with poly(A)+ RNA from 21C11 cells contained 36 colonies that hybridized with the MuLT BamHI fragment. A full-length cDNA has been isolated, sequenced, expressed in COS-1 cells and used to map MuLT to mouse chromosome 17. The sequence and structure of the MuLT gene has been determined. MuLT cDNA has been used to analyse mRNA expression in several L3T4+ and Lyt-2+ (CD8+-equivalent) T cell clones activated with antigen, mitogen, or antibody to the T cell receptor. LT is expressed by both class I- and class II-restricted T cells. The mechanism of killing by both LT and the functionally related molecule TNF-alpha includes the induction of DNA fragmentation in the target cell.

Isolation and identification of an alpha 2 subclass lymphotoxin (LT) subunit from the high-molecular-weight (complex) human LT class

The high-molecular-weight (MW) complex class (MW = greater than or equal to 200,000) of lymphotoxin (LT) may be involved in the process of lymphocyte-mediated cytolysis. This LT class was produced by concanavalin A-stimulated human adenoid and tonsil lymphocytes in vitro and purified approximately 1000 fold by a scheme employing lectin affinity chromatography on concanavalin A-Sepharose, negative hydrophobic affinity chromatography on phenyl-Sepharose, and molecular sieving on Ultrogel AcA 44. A cell-lytic subunit, termed C-alpha toxin, was dissociated from this partially purified complex LT (Cx) preparation. The identification of C-alpha toxin as an alpha 2 LT form was established by its comigration with the alpha 2 LT form during native polyacrylamide gel electrophoresis (PAGE), molecular sieving, and isoelectricfocusing. In addition, C-alpha toxin was neutralized by antiserum raised against purified alpha 2 LT, and the latter was neutralized by antiserum raised against C-alpha toxin. Furthermore, preliminary evidence indicates that both alpha 2 LT and C-alpha toxin are composed of peptides with apparent MW of approximately 69,000. Additional data indicate the Cx LT form is composed of alpha 2 LT and other subunits.

Lymphotoxins - a multicomponent system of growth inhibitory and cell-lytic glycoproteins

Activated lymphocytes from experimental animals and man can release materials, termed lymphotoxins , which cause growth inhibition and cell lysis in vitro. These molecules, from human lymphocytes, are glycoproteins which can be divided into five molecular weight classes. These forms are heterogeneous, for each MW class can be further subdivided into multiple charge subclasses. It is now clear certain MW classes are interrelated and form a system of cell toxins. The larger classes (greater than 140,000 d) are associated with nonclassical antigen-binding receptors (R), which can be of T cell origin. The smaller forms (less than 90,000 d) do not express R function and are derived from the larger forms, possibly by enzymatic action. Two MW classes, one receptor-associated and one non-receptor associated, have been purified to homogeneity and their peptide composition is being studied. Functional studies reveal the larger MW forms derived from alloimmune cell populations; can induce selective and nonselective destruction of cells in vitro. Antibodies which inhibit the in vitro cell lytic ability of various human LT forms can block different classes of human lymphocyte cell killing reactions in vitro.

Inhibition of human NK-induced cell lysis and soluble cell-lytic molecules with anti-human LT antisera and various saccharides

The present study examines and compares the cytolysis of K-562 and MOLT-4 cells mediated by human natural killer (NK) cells from fresh peripheral blood and lymphotoxins (LT) derived from human lymphoid cell populations after lectin stimulation in vitro. Lymphotoxins were obtained from 5-hr concanavalin A (Con A)-restimulated human peripheral blood lymphocytes (PBL) which were precultured for 5 days in medium and fetal calf serum or with allogeneic human B-lymphoid cell lines. Two classes of probes were employed in both direct (cell) and indirect (supernatant) induced target-cell lysis: (a) various saccharides and (b) antibodies reactive with human LT forms. Two sugars, N-acetylglucosamine and alpha-methylmannoside, were able to inhibit direct cell lysis of both MOLT-4 and K-562 target cells. However, saccharide inhibition was distinct for each type of target even when effector cells were obtained from the same donor. These same saccharides were also able to inhibit 20-30% of the total LT activity in a supernatant for L-929 cells and 50-90% of the lytic activity on MOLT-4 cells. Anti-human F(ab')2 (IgG) and rabbit anti-alpha 2 LT sera blocked direct cell lysis of MOLT-4 and K-562 targets in 50% of the experiments. The anti-alpha 2 LT serum only recognizes a portion of the LT forms in these supernatants. These results reveal that, while both direct and indirect cell lysis are complex phenomena, they may both occur in some cases by a common mechanism(s).

Identification of beta-lymphotoxin as the predominant molecular class of in vitro and in vivo Syrian hamster lymphotoxin

The molecular class of Golden Syrian hamster lymphotoxin produced in vitro and in vivo was determined by size-exclusion high-performance liquid chromatography using silica-based protein separation columns eluted with a 0.1 M sodium phosphate, pH 7.4 buffer containing 0.1% Mr 4000 polyethylene glycol. Lymphotoxin cytolytic activity was quantitated in the column effluent by measuring the ability of the fractions to lyse alpha-L929 cells as indicated by [3H]TdR release. Lymphotoxin activity induced by an 8- or 24-hr or 5-day phytohemagglutinin stimulation of peritoneal leukocytes, by 24-hr phytohemagglutinin-coated alpha-L929-cell stimulation of peritoneal leukocytes, or by 24-hr phytohemagglutinin stimulation of spleen cells occurred in the Mr range of 20,000-56,000, with major components in the 35,000-50,000 beta-lymphotoxin region. No activity was present in the complex (greater than 200,000) region and only minimal activity was detectable in the alpha (70,000-160,000) and gamma (12,000-20,000) regions. In vivo-induced lymphotoxin, obtained by peritoneal lavage 48 hr after intraperitoneal administration of phytohemagglutinin, was entirely beta-lymphotoxin and was not detectable in the plasma. Lymphotoxin produced in vitro and injected simultaneously with the gamma-emitting radionuclide 99mtechnetium, inhibited in vivo development of radiation-induced transplacental carcinogenesis. Thus, Syrian hamster lymphotoxin with antitumor activity consists of glycoproteins with isoelectric points of 4.8-5.2, Mr of 20,000-56,000, and major in vitro and in vivo forms in the beta-lymphotoxin range.

Rapid separation of biologically active Syrian hamster lymphotoxin in high yield by size-exclusion high-performance liquid chromatography

A rapid, simple and reproducible high-performance liquid chromatography (HPLC) procedure is described for the partial purification of biologically active Syrian hamster lymphotoxin. Lymphotoxin, a series of lymphocyte derived immunological glycoprotein hormones with molecular weights of 20,000 to 56,000, has been partially purified by a single-step HPLC procedure or by a two-step isoelectric focusing-HPLC procedure. The HPLC separation method uses silica-based protein separation columns eluted with a 0.1 M sodium phosphate, pH 7.4, 0.1% polyethylene glycol 4000 buffer at a flow-rate of 0.5 ml/min at room temperature. Nearly complete recoveries of biologically active lymphotoxin can be obtained with as much as a 13-fold purification in less than 1 h.