Immunohistochemical localization of C9 neoantigen and the terminal complement inhibitory protein CD59 in human endometrium

Blood SC5b-9 complement levels increase at parturition during term and preterm labor

We explored the hypothesis that complement, an innate and adaptive immune effector, is active in the plasma of parturient women and is deposited on fetal membranes collected after delivery. A cross-sectional study was designed to evaluate complement activity at parturition. Pregnant women (n = 97) between 15 and 41 years of age were enrolled in a hospital protocol during the perinatal period to assess both SC5b-9 complement activity in blood and complement deposition on fetal membranes during parturition. Soluble SC5b-9 complement activity in plasma fractions was measured using a standard enzyme-linked immunosorbent assay (ELISA) that included specific anti-complement antibodies. Complement deposition on membranes was analyzed using immuno-dot blots and immunohistochemistry. Soluble SC5b-9 complement complex levels were increased in the plasma of women during term labor (TL; median 3361; range 1726-5670 ng/mL), preterm labor (PL; median 2958; range 1552-7092 ng/mL), and preterm premature rupture of membranes (PPROM; median 2272; range 167-6540 ng/mL) compared with pregnant women who were not in labor (P; median 1384; range 174-4570 ng/mL; P < 0.001, Kruskal-Wallis test). Active complement, as assessed by the C9 neo-antigen in C5b-9 complexes, was deposited on fetal membranes, with no difference between term and preterm delivery. The deposition of active complement on fetal membranes was confirmed by immunohistochemistry. Women who underwent non-labor-indicated Cesarean sections did not exhibit complement deposition. Soluble SC5b-9 complement complex levels increased in the plasma of women during parturition, and complement C5b-9 complexes were deposited on fetal membranes.

Alternative roles for CD59

CD59 was first identified as a regulator of the terminal pathway of complement, which acts by binding to the C8/C9 components of the assembling membrane attack complex (MAC), to inhibit formation of the lytic pore. Structurally, CD59 is a small, highly glycosylated, GPI-linked protein, with a wide expression profile. Functionally, the role of CD59 in complement regulation is well-defined but studies have also shown clear evidence for signalling properties, which are linked to its glycophosphatidyl inositol (GPI) anchor and its location within lipid rafts. Cross-linking of CD59 using specific monoclonal antibodies drives both calcium release and activation of lipid-raft associated signalling molecules such as tyrosine kinases. These observations clearly show that CD59 exhibits roles independent of its function as a complement inhibitor. In this review, we examine the progression of research in this area and explore the alternative functions of CD59 that have recently been defined.

Loss of CD59 expression in breast tumours correlates with poor survival

CD59 (protectin), a phosphatidylinositol-anchored glycoprotein, is a member of the cell membrane-bound complement regulatory proteins that inhibits the formation of the terminal membrane attack complex (MAC) of complement. In this study, the expression of CD59 was evaluated in 520 breast carcinomas from patients with a mean follow-up of 87 months. This expression was correlated with clinicopathological features and patient survival. Marked variation in the intensity of CD59 expression, which correlated with histological grade and Nottingham prognostic index (NPI), was found, with higher expression of CD59 found more often in well and moderately differentiated tumours and those of good prognosis (NPI < or = 3.4). In contrast, high grade and poor prognosis (NPI > 5.4) carcinomas significantly demonstrated lack of CD59 expression (p < 0.001). Moreover, it was found that the percentage of CD59-positive cells correlated significantly with patient survival, ie patients with a high percentage of positive cells (>50%) had a better overall survival (p = 0.006). A correlation was also found between the percentage of CD59-positive cells and tumour type and also the development of distant metastases. No association was found between either the intensity or the percentage of cells expressing CD59 and vascular invasion, lymph node stage, tumour size, patient age or menopausal status. In multivariate analysis, CD59 percentage positivity was of independent prognostic significance with grade and lymph node stage. These findings indicate that loss of CD59 may offer a selective advantage for breast cancers, resulting in more aggressive tumours and conferring a poor prognosis for patients.

Human complement regulator expression by the normal female reproductive tract

BACKGROUND

The membrane-associated proteins that regulate human complement activation are ubiquitously expressed and function cooperatively to protect cells from autologous complement damage. For classical and alternative pathways, the primary regulators at the stage of C3 proteolysis and deposition are membrane cofactor protein (MCP; CD46) and decay-accelerating factor (DAF;CD55), whereas protectin or CD59 regulates terminal component assembly. There is increasing awareness in reproductive, tumor, and transplantation immunology of the conventional and non-complement roles of these proteins. The human reproductive system may serve as a model of the non-complement functions.

METHODS

We performed immunohistochemical analyses of multiple normal ovaries, fallopian tubes, cervices, and uterine corpi by using well-characterized monoclonal antibodies to provide a detailed, direct comparison of complement regulator expression.

RESULTS

Membrane cofactor protein was diffusely and strongly expressed on all epithelia and vascular endothelium and was the predominant regulator on oocytes. In contrast, decay-accelerating factor had variable expression in intensity and distribution on epithelia and was notably absent on certain epithelia and oocytes. It was the only regulator present on the connective tissue between muscle bundles in the myometrium and the cervix and was found on most stroma. CD59, although staining intensity varied, was present on virtually all epithelia, vascular tissue, and stroma.

CONCLUSIONS

Distinct reproducible patterns of complement regulator expression are found throughout the female reproductive tract. Differential expression on certain epithelia and oocytes may suggest non-complement activities. This comprehensive study should provide a basis for further characterization of pathological tissues and mechanisms of cellular localization.