Macrophage and microglial responses to cytokines in vitro: phagocytic activity, proteolytic enzyme release, and free radical production.

Certain cytokines are believed to play a key role in the development of autoimmune demyelinating diseases. Little is known, however, about the effects of these cytokines in the regulation of the key event in myelin destruction, the phagocytosis of myelin by phagocytic cells. We investigated the effects of certain cytokines and growth factors on cultured peritoneal macrophages and microglia in respect to their various functions, phagocytosis, secreted proteolytic activity, and oxidative activity. Interferon-gamma (IFN-gamma), tumor necrosis factor-alpha (TNF-alpha), and lipopolysaccharide (LPS), all proinflammatory factors, actually decreased (IFN-gamma and LPS), or had no effect (TNF-alpha) on myelin phagocytosis by macrophages, but substantially increased phagocytic activity by microglia. Surprisingly, interleukins 4 and 10 (IL-4 and IL-10), considered to be downregulating cytokines, increased phagocytic activity by macrophages, while with microglia, IL-4 had no effect, but IL-10 almost doubled myelin phagocytosis. Transforming growth factor-beta (TGF-beta) had no significant effect on either cell. These cytokines did not affect proteolytic secretion in microglia, while IFN-gamma and LPS induced a doubling of the secreted proteases. This proteolytic activity was almost completely suppressed by calpain inhibitors, although some gelatinase appeared to be present. Microglia exerted much more oxidative activity on the membranes than macrophages, and granulocyte-macrophage colony stimulating factor (GM-CSF) and interleukin 1beta (IL-1beta) significantly increased microglial oxidative activity. The pattern of responses of macrophages and microglia to the cytokine types indicate that in cytokine-driven autoimmune demyelinating disease, microglia may be the more aggressive cell in causing tissue injury by phagocytosis and oxidative injury, while infiltrating macrophages may produce most of the proteolytic activity thought to contribute to myelin destruction.

Effects of phorbol myristate acetate (PMA) on functions of macrophages and microglia in vitro.

Peripheral macrophages infiltrating the central nervous system and resident microglia phagocytize myelin in cell-mediated demyelinating diseases, including experimental autoimmune encephalomyelitis and multiple sclerosis. A cascade of cytokines is believed to modulate the immunological sequence of events occurring in these conditions, and several of these mediate their effects through the protein kinase C pathway. Therefore, we compared the effects of phorbol myristate acetate (PMA), an activator of protein kinase C, on various functions of cultured macrophages and microglia. PMA at moderate concentrations induced apoptosis in macrophages, and this process appeared to be increased in the presence of myelin. In contrast, microglia were activated by PMA, and greatly increased their phagocytosis of myelin. Control macrophages released a considerable amount of proteolytic activity into the medium, as measured by the breakdown of myelin basic protein, and in the process of undergoing apoptosis from PMA-treatment, even higher amounts were released. The enzyme activity in control macrophage medium was inhibited mainly by PMSF and calpain inhibitors, while that from PMA-treated macrophages was inhibited by calpain inhibitors only. An ICE inhibitor was ineffective in inhibiting activity in medium from PMA-treated cells undergoing apoptosis. Medium from microglia contained very little proteolytic activity, and this was not increased by PMA. Cultured macrophages showed little evidence of oxygen free radical release as measured by the TBARS procedure, and PMA had no effect. Microglia, on the other hand, produced higher levels of reactive oxygen species, with a further increase of 18% by PMA. Thus major functions of these phagocytic cells appear to be modulated by the protein kinase C pathway, although the two cell types show very different responses to an activator of this signal.

GFAP mRNA fluctuates in synchrony with chronic relapsing EAE symptoms in SJL/J mice.

Activation of astrocytes and hypertrophy of their processes is a result of a number of pathological conditions in the central nervous system. Astrocytic gliosis is especially prominent in multiple sclerosis (MS), where astrocytic fibers form a dense matrix around demyelinated axons. Experimental allergic encephalomyelitis (EAE), a laboratory model for MS, is also accompanied by astrocytic hyperactivity. We have previously shown the formation of plaque-like structures which stain heavily for glial fibrillary acidic protein (GFAP) in the brains and spinal cords of SJL/J mice after several episodes of chronic relapsing EAE (Smith and Eng: J Neurosci Res 18:203, 1987). To further investigate the mechanisms of this phenomenon, we have measured the levels of mRNA for GFAP throughout the course of three episodes and recoveries of EAE in the SJL/J mouse. Mice were immunized with spinal cord homogenate and subsequently developed EAE. After recovery they were again immunized at appropriate intervals, resulting in successive episodes of EAE, with partial or complete recovery between the paralytic stages. At appropriate times in the course of the different stages of EAE, spinal cords were dissected and RNA was prepared from each spinal cord. RNA was analyzed by Northern blots to determine the levels of mRNA for GFAP and, as a control for the 70 kDa neurofilament (NF-L). With the onset of the first EAE episode GFAP mRNA in spinal cords from animals with mild symptoms increased to sixfold the control level (P < 0.02) and to 20-fold in those with paralysis (P < 0.01).(ABSTRACT TRUNCATED AT 250 WORDS)

Enzymatic regulation of glycoprotein synthesis in peripheral nervous system myelin.

The enzyme UDP-N-acetylglucosamine: dolichyl phosphate, N-acetylglucosamine-1-phosphate transferase initiates the synthesis of the oligosaccharide chain of complex-type glycoproteins. In view of the high content of glycoprotein in peripheral nerve myelin, the properties of this enzyme, its changes with age, and the effect of the specific inhibitor tunicamycin were investigated. The enzyme activity in rat peripheral nerve homogenate was completely dependent on the presence of exogenous dolichyl phosphate as well as Mg2+ and a detergent (Triton X-100) and was also greatly stimulated by a high salt concentration (0.4 M KCl) and AMP. The highest specific activity was present in the postmitochondrial membranes. The specific activity in postmitochondrial membranes in the presence of exogenous dolichyl phosphate reached a maximum at 17 days and remained relatively high throughout development, up to 2 years of age, but the activity was much lower when dolichyl phosphate was not added. This indicates that the enzyme level does not decrease with age, but that the content of the lipid cofactor may limit glycoprotein synthesis in vivo. Tunicamycin (5 micrograms) was injected intraneurally into 24-day-old rat sciatic nerve, and the enzyme was assayed from 1 to 24 days after injection. The specific activity of the transferase remained at low levels (5-40% of the level in control nerve) in most injected nerves assayed throughout this postinjection period. A protein previously identified as the unglycosylated P0 protein was synthesized in vitro by the tunicamycin-injected nerve and could be demonstrated to be incorporated into myelin in large amounts at 2 days and in small amounts at 6 days after injection.(ABSTRACT TRUNCATED AT 250 WORDS)

DNA changes in spinal cords of rats with experimental allergic encephalomyelitis.

DNA levels were measured in the spinal cords of Lewis rats during the development of and recovery from experimental allergic encephalomyelitis (EAE). Spinal cord DNA was first increased 11 days after immunizing the rats with guinea pig myelin and rose to levels four times that of the Freund's adjuvant controls at day 14, then subsided after day 22. Spinal cord DNA was still 150% of control levels 60 days after immunization. These DNA changes were compared with fluctuations in spinal cord acid proteinase in the same animals. Acid proteinase activity in EAE spinal cord increased later than the rise in DNA and attained a level of 170% of control at days 15-17, then subsided. Spinal cord DNA was higher in rats immunized with whole myelin than in those administered equivalent amounts of purified myelin basic protein. Furthermore DNA was higher in spinal cords of rats immunized with a larger dose of myelin (1.0 mg) than with a lower amount (0.5 mg). Various protease inhibitors including pepstatin, nitrophenyl p-guanidino benzoate, polylysine, and dipropionyl rhein, previously shown to protect Lewis rats against EAE, suppressed the increase of DNA in the spinal cord. Measurement of DNA increases in the spinal cord of EAE animals provides a convenient reproducible measurement of the severity of inflammation in the CNS and provides an objective criterion for assessment of the efficacy of various agents screened as possible therapeutic treatment for multiple sclerosis.

Immunocytochemical staining for glial fibrillary acidic protein and the metabolism of cytoskeletal proteins in experimental allergic encephalomyelitis.

Spinal cord sections from Lewis rats with acute experimental allergic encephalomyelitis (EAE) showed greatly increased staining of astrocytes when stained immunocytochemically for glial fibrillary acidic protein (GFAP). Fibrous processes in white matter were heavily stained early in the course of the disease when paralysis was first evident (10-12 days after injection of guinea pig spinal cord myelin), then protoplasmic astrocytes were stained in the gray matter and became more heavily stained at 20 days post-injection. The stained astrocytes were evenly distributed throughout the tissue, and did not correspond to the sites of the lesions. Spinal cord slices of control and EAE rats were incubated with [3H]amino acids, then cytoskeletal proteins were prepared in an enriched fraction, separated by sodium dodecyl sulfate-polyacrylamide gel electrophoresis (SDS-PAGE), and the protein bands counted for radioactivity. In the EAE rat all cytoskeletal proteins, including the neurofilaments, vimentin, microtubules, GFAP and actin, showed increased uptake of radioactive amino acids. Immunoprecipitation of GFAP with specific antiserum showed increased radioactivity in the complex beginning at day 10 when cellular infiltration was beginning in the EAE animals. As the disease became acute, the radioactivity in the immunoprecipitated GFAP increased, in some cases to very high levels, then by day 18 when recovery was underway, the radioactivity had fallen to normal levels. Possible agents causing metabolic activation of protein synthesis in EAE animals include stimulating substances elaborated by infiltrating lymphoid cells, and the generalized edema accompanying the demyelinative condition. The activation of GFAP protein staining and metabolism in EAE might serve as a model for the activated growth of astrocyte processes which cause the severe gliosis seen in multiple sclerosis.