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Lectin-Induced Mitogenesis

In 1960 Nowell added PHA to a blood sample to agglutinate erythrocytes and thus encourage their removal and noticed to his annoyance that the lymphocytes had also been affected. He had discovered the mitogenic effect of PHA (and many other lectins) which was to be the key to the explosion of knowledge about lymphocyte physiology. Lectins are probably the best biologic response modifiers (outside of monoclonal antibodies) found in nature.

Hemagglutinating properties are not necessary for a lectin to possess mitogenic activity. Many mitogens are "lectins" only if we enlarge the category to include monovalent molecules with high carbohydrate affinity. Paradoxically, any plant polysaccharides can be thought of as "reverse lectins" i.e. their sugars bind lectin-like receptors on the call. This has been demonstrated for polysaccharides isolated from Thuja occidentales, which show high mitogenic activity that is blocked by anti-interleukin I antibodies, This proves that plant polysaccharides are definite biologic response modifiers. Other polysaccharides from higher plants such as Baptisia tinctoralis (heteroglycans) or Angefica acutiloba ("Angelica immunostimulating polysaccharide") and the fungii Basidlomycetes (lentinen, schizophylan, pachymaran and krestins) have also shown mitogenic and respose modifying activity.

How mitogens work is still imperfectly understood. Con-A has been shown to induce microtubule assembly in polymorphonuclear leukocytes. Lectins have been shown to cause early changes in cytoplasmic free Ca2+ and influence the lymphocyte membrane potential. Both Con-A and PHA were studied as to their effect on lymphocyte glycosyltransferase activty. The investigators found that this enzyme, associated with increased transport activity of sialic acids, galactose and NAG was stimulated by Con-A but not by PHA. Thus the mitogenic effects of lectins on lymphocytes is not constant.

Lymphocytes in mitosis are almost never found in peripheral blood, but they were observed frequently in the blood smears of children who has eaten the North American shrub called pokeweed (Phytolacca amer.) Pokeweed mitogen is one of the few lectins that stimulates B lymphocytes as well as T lymphocytes. In vitro it triggers the production of IgE as well as other antibody isotypes. The discovery that grass pollen apparently share a common lectin perhaps offers a clue as to why pollen so often provoke allergy.

Lymphoid cells from patients with chronic lymphatic leukemia bind less PHA than do normal cells, and react poorly to the mitogenic activity of this and other lectins. B lymphocytes stimulated by lectins are capable of synthesizing antibodies; T- lymphocytes may be turned into "killer cells" that destroy any foreign cells that they contact. Many subpopulations of lymphocytes are specifically stimulated by particular lectins. Separating mouse thymocyte populations into two groups, one that was agglutinated by peanut lectin and one that is not. The thymocyte population found to not be agglutinated by the lectin was found to resemble the adult circulating lymphocytes. Only this population of thymic lymphocytes has a high sialic acid content on its membrane, leading researchers to speculate that the at

Blastogenic effects

The lectin most studied in humans as regards to mitogenic effects is pokeweed mitogen (PWM), isolated from Phytolacca americana. Phytolacca lectin is one of the rare lectins which is mitogenic for both T and B lymphocytes. Recent studies on the plant show that salt water extracts of the plant yield five separate lectins, designated Pa-l through Pa-5. Pa-1 seems to be the only heamagglutinating lectin, and is powerfully mitogenic. Pa-2 and Pa-4 are the predominant mitogens in the roots. Pa-1 is mitogenic for both B and T cells, while the other four lectins are only mitogenic for T cells. Interestingly, PWM blastogenesis is inhibited by other lectins such as WGA. Benincasa cerifera, used in Sino-chinese medicine as an antinflamatory diuretic, was shown to contain a powerful anti-tumor mitogen termed "B. cerifora mitogen" (BCM). Salt water extracts of the seed were shown to contain B cell mitogenic, adjuvant active and antitumor active substances.

Wheat germ agglutinin (WGA) induces proliferation of T-cell colony forming units and growth factor production. PHA can induce the acquisition of T cell surface markers in peripheral blood in the absence of the normal maturation controls of the thymus. Other studies showed that this occurred only in high IL-1 environments. This was shown to be produced by "mitogen induced erythroid burst promotion" due to monocyte blastogenesis produced by Con-A. Interestingly PWM did the exact opposite (suppressed erythroid burst activity), which could account for the anti-inflamatory activity traditional ascribed to the plant. Human peripheral blood lymphocytes precultured with lipopolysaccharide from E.Coli (LPS) were shown to have a greatly enhanced blastogenic response when pokeweed mitogen was added to the suspension.

Injections of lentil lectin into the knee joint cavity of non-sensitized rabbits resulted in the development of arthritis which was indistinguishable morphologically from rheumatoid.

In a rather perverse way "negative-blastogenesis" can also be produced by using appropriate sugar molecules to "suppress the suppressors". Several sugars have been shown to selectively do this including mannose and fucose. Lectin Induced blastogenesis may have some impact In the myeloproliferative disorders. Hodgkin disease cells have been shown to elaborate an agglutinating lectin on their surfaces.

Immunosuppresive effects

Since blastogenesis can also occur in suppressor -T cell populations, it is quite feasible that significant suppression of graft versus host responses in tissue transplants can be accomplished by the use of lectins. Significant studies are now under way at Stanford University showing that lectins can be used exclusively to maintain transplants in animals for up to two years. Lentil lectin induces striking transplant tolerance In both mice and humans. Peanut agglutinin has been used to isolate suppressor T-cells in vivo, these having been first Induced by Con-A. Tomato lectin has been shown to inhibit the transformation of peripheral lymphocytes challenged by recall antigens, and actually suppressed spontaneous DNA synthesis. The inhibition of lymphocyte transformation was not stopped by exogenously added Interleukin 1 and/or Interleukin 2, even at extremely high concentrations. This could be significant as the average American diet results in the ingestion of at least 200 mg. of tomato lectin annually, with vegetarians probably ingesting a far greater amount.

PHA has been shown to suppress experimental autolmmune thyroiditis in mice for up to 7 week. Electrolectin from the electric eel (Electrophorus electricus) was shown to prevent and effectively treat experimental auto-immune myastenia gravis in rabbits, considered a good model for the human disease myastenia gravis. Administration of electrolectin to the afflicted rabbits lead in all cases to complete recovery, presumably through modulation of the suppressor cell activity directed against acetylcholine receptor protein self antibodies.

Chinese bitter melon lectin (Mornordica charantia) has been shown to possess potent immunomodulatory activity. "Locoweed" and several species of Astragalus and Oxytropis, when fed to yearling ewes, resulted in a gradual decrease in total leukocyte and peripheral lymphocyte blood levels.