TNF-Rs

The distribution of the two TNF-Rs varies, depending on the cell type. The majority of TNF-Rs on epithelial cells are TNF-Rls, whereas TNF-R2s are predominantly found on myeloid and lymphoid cells (Hohmann et al., 1989 Ware et al., 1991). Other cell types, such as neutrophils, may express similar amounts of both TNF-Rs (Porteau et al., 1991). Taken together, these findings suggest that the effects of TNF depend, in part, on the specific activity and composition of TNF-Rs on the target cells. 

The mRNA and protein expression of TNF-Rs can be modulated. TNF-R mRNA may be either down- or up-regulated. Surface TNF-Rs can be increased or decreased in number. A reduction may occur through internalization or shedding, the formation of soluble 30- to 40-kDa fragments (soluble TNF-Rs) from the proteolytic cleavage of the surface receptor extracellular domain (Olsson et al., 1993). Modulation of TNF-R mRNA and protein 

TNF-binding studies indicate that in the absence of active ligand or modulatory agents, surface TNF-Rs are continuously internalized and then degraded inside lysosomes (Tsujimoto and Vilcek, 1987 Watanabe et al., 

1993) and nonimmune (Taylor, 1994) cell types, indicating that shedding, like internalization, may be associated with physiological TNF-R turnover. 

The effect of IFN-y on TNF-Rs is complex. While IFN-y induces cells to synthesize TNF and IL-1 and thus could down-regulate TNF-Rs, IFN-y actually increases the number of surface TNF-Rs without a significant change in binding affinity (Aggarwal et al., 1985 Tsujimoto and Vilcek, 1987). This effect of IFN-y is similar for both receptor types in many myeloid and epithelial cells (Pandita et al., 1992). However, in mouse peritoneal M (), 

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Shedding does not always lead to diminished cell surface receptor. IL-4 and IL-10 are two cytokines with antiinflammatory effects on several cell types. IL-4 alone has no effect on surface TNF-R density but does induce shedding in rheumatoid synovial fibroblasts (Taylor, 1994), indicating that receptors are rapidly replaced. In cultures of peripheral blood mono-nucleated cells, 2-hr lL-10 treatment reduces surface TNF-R expression as detected by flow cytometry, but enhanced reexpression of TNF-Rs occurs by 48 hr (Leeuwenberg et al., 1994). Interestingly, IL-10 activates TNF-Rl and TNF-R2 shedding from these cells, whereas IL-4 has no such effect (Leeuwenberg et al., 1994). 

PTX has been studied recently for its effect on TNF-Rs. Although PTX blocks TNF production by normal human dermal fibroblasts, it does not alter the number of surface TNF-Rs, or constitutive and TNF-induced levels of TNF-Rs mRNA in vivo (Berman et al., 1992). However, PTX reduces TNF-R shedding when it is administered in vivo to mice (Bemelmans et al.,... 

Soluble fragments of TNF-Rs were first referred to as TNF-binding proteins, having been discovered in the biological fluids of health volunteers, postmenopausal women, and patients with cancer or chronic renal disease (Olsson et al., 1993). These soluble forms of TNF-Rs are immunologically distinct from one another but demonstrate functional cross-reactivity with the surface receptors (Engelmann et al., 1990b). 

The discovery of these soluble TNF-R fragments has preceded the understanding of how they are generated. Since molecular studies have not been able to identify mRNA specific for the soluble TNF-R forms, and amino acid sequences of the soluble forms are identical to those of the extracellular domains of surface TNF-Rs (Nophar et al., 1990), it has been concluded that the fragments are released from the cleavage of surface receptors and are not produced from alternatively spliced surface TNF-R transcripts. Additional supporting evidence for this conclusion is that TNF-binding proteins... 

Therefore, these findings have led to the hypothesis that the generation of soluble TNF-Rs is the result of proteolytic cleavage of the extracellular domains of membrane-bound TNF-Rs. 

Because we believe TNF and NO are important in lesion formation in MS, we have become interested in the modulation of surface and soluble TNF-Rs in the CNS. Here, we have chosen to examine the paradigms previously established in immune cells in human glial cell cultures in an effort to relate the interrelationship and regulation of these entities. 

While functional studies indicate that astrocytes possess TNF-Rs, the specific characterization of TNF-R 1 and TNF-R2 on human astrocytes under constitutive and activated conditions has not been well studied to date. The number of surface TNF-R 1 and TNF-R2 on human astrocytes has not been determined quantitatively there may be a differential expression of the two receptor types on astrocytes, as there is on epithelial and myeloid cells. Agonistic TNF-R antibodies, such as anti-TNF-Rl, mimic the effects of TNF by inducing normal human astrocytes to synthesize cytokines, such as IL-6 and monocyte chemoattractant protein 1, and to undergo growth modulation (Barna et al., 1993, 1994). Since anti-TNF-Rl but not anti-TNF-Rl antibodies induce an effect on normal human astrocytes, and only TNF-Rl transcripts are strongly detected in untreated fetal and adult astrocytes, it has been suggested that TNF-Rl is the predominant constitutive receptor type on human astrocytes (Barna et al., 1993 Tada et al., 1994). 

It is clear that TNF production and TNF-R shedding may be independently controlled (Brakebusch et al., 1992). Because of this, we also tested astrocyte supernatants for the presence of TNF and NO in order to correlate their production with increased concentrations of soluble TNF-R, since previous TNF-R shedding studies have not measured soluble TNF, TNF-Rs, and NO under the same stimulating conditions. Both TNF and NO in... 

VI. Potential Roles of TNF-Rs in Association with Astrocyte TNF and NO Production ... 

Nophar, Y., Kemper, O., Brakebusch, C., Engelmann, H., Zwang, R., Aderka, D., Holt-mann, H., and Wallach, D. (1990). Soluble forms of tumor necrosis factor receptors (TNF-Rs). The cDNA for the type I TNF-R, cloned using amino acid sequence data of its soluble form, encodes both the cell surface and a soluble form of the receptor. EMBO ]. 9, 3269-3278. 

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