136. SPECIFICITY STUDIES ON THE IMMUNOSUPPRESSIVE AND CYTOTOXIC ACTIVITIES OF LYSOPHOSPHATIDYLCHOLINES (LPCS) OF GONADAL ORIGIN

Abstract

The immunosuppressive activity of ovarian follicular fluid and testicular interstitial fluid is due to the presence of several LPCs, but the specificity of this inhibition is a potential source of controversy, as these molecules also possess lytic activity. In the following study, we compared the immunosuppressive and cytotoxic activities of the two most abundant gonadal LPCs, 1-palmitoyl-sn-glycero-3-phosphocholine (16:0aLPC) and 1-oleoyl-sn-glycero-3-phosphocholine (18:1aLPC), together with a number of related lysophospholipids (LPs), using a T-cell activation inhibition assay and an ovarian granulosa cell viability assay. Both the immunosuppressive and cytotoxic activities of the LPCs were blocked by serum (>5%) and serum albumin (>5mg/ml) in vitro.  In the absence of serum proteins, the most immunosuppressive LPCs were 16:0aLPC, 18:0aLPC, 18:1aLPC and platelet activating factor (PAF; 1-O-palmitoyl-2-O-acetyl-sn-glycero-3-phosphocholine) with IC₅₀ values of 1.2-4.3 μM. Curiously, PAF was the LPC most cytotoxic to granulosa cells (IC₅₀ 10 μM). The other immunosuppressive LPCs exhibited cytotoxicity within the range of 40-50 μM, i.e. at doses 10–50-fold higher than their immunosuppressive concentrations. Comparison of LPs of different structures indicated that optimal immunosuppression is related to a phosphocholine, but not serine, ethanolamine or phosphate group, at sn-3, and an ester- or ether-linked fatty acid of chain length C16-C18 at sn-1. Acetylation of sn-2, as in PAF, had only minor effects on immunosuppressive activity, but greatly increased cytotoxicity. These data establish that inhibition of activated T-cells is not a direct consequence of the cytotoxicity of these molecules, although some structural features that contribute to lytic activity, such as fatty acid chain length, overlap with the ability to confer immunosuppression. On the basis of these data, we propose that the effects of LPCs on T-cell proliferation may not be mediated by a specific lock-and-key receptor, but rather by a direct interaction with the cell membrane at concentrations significantly below their lytic concentrations.