Fine specificity of the autoimmune response to the Ro/SSA and La/SSB ribonucleoproteins

The fine specificity of the Ro and La proteins has been studied by several techniques. In general, there is agreement in a qualitative sense that autoantibodies bind multiple epitopes. For some specific antibody binding, different studies agree quantitatively, for instance, the binding of the carboxyl terminus of 60-kd Ro as described by 2 studies using different techniques and the presence of an epitope within the leucine zipper of 52-kd Ro. In addition, there is general agreement about the location of a prominent epitope at the RRM motif region of the La molecule. On the other hand, the many specific epitope regions of the molecules differ among these studies. These discrepancies are likely the result of using different techniques, sera, and peptide constructs as well as a result of inherent advantages and disadvantages in the individual approaches. Several theories concerning the origin of not only the antibodies, but also the diseases themselves, have been generated from studies of the fine specificity of antibody binding. These include a theory of a primordial foreign antigen for anti-Ro autoimmunity, molecular mimicry with regard to La and CCHB, as well as the association of anti-Ro with HLA. These remain unproven, but are of continuing interest. An explanation for the association of anti-60-kd Ro and anti-52-kd Ro in the sera of patients has sprung from evaluating antibody binding. Data demonstrating multiple epitopes are part of a large body of evidence that strongly suggests an antigen-driven immune response. This means that the autoantigens are directly implicated in initiating and sustaining autoimmunity in their associated diseases. A number of studies have investigated the possibility of differences in the immune response to these antigens in SS and SLE sera. While several differences have been reported, none have been reproduced in a second cohort of patients. Furthermore, none of the reported differences may be sufficiently robust for clinical purposes, such as distinguishing between SS with systemic features and mild SLE, although some might be promising. For instance, in at least 3 groups of SLE patients, no binding of residues spanning amino acids 21-41 of 60-kd Ro has been found. Meanwhile, 1 of those studies found that 41% of sera from patients with primary SS bound the 60-kd Ro peptide 21-41. Perhaps future studies will elaborate a clinical role of such a difference among SS and SLE patients. Study of the epitopes of these autoantigens has, in part, led to a new animal model of anti-Ro and anti-La. Non-autoimmune-prone animals are immunized with proteins or peptides that make up the Ro/La RNP. Such animals develop an autoimmune response to the entire particle, not just the immunogen. This response has been hypothesized to arise from autoreactive B cells. In another, older animal model of disease, the MRL-lpr/lpr mouse, B cells have recently been shown to be required for the generation of abnormal, autoreactive T cells. Thus, there are now powerful data indicating that B cells that produce autoantibodies are directly involved in the pathogenesis of disease above and beyond the formation of immune complexes. Given that the autoreactive B cell is potentially critical to the underlying pathogenesis of disease, then studying these cells will be crucial to further understanding the origin of diseases associated with Ro and La autoimmunity. Hopefully, an increased understanding will eventually lead to improved treatment of patients. Progress in the area of treatment will almost surely be incremental, and studies of the fine specificity of autoantibody binding will be a part of the body of basic knowledge contributing to ultimate advancement. In the future, the animal models will need to be examined with regard to immunology and immunochemistry as well as genetics. The development of these autoantibodies has not been studied extensively because upon presentation to medical care, virtually all patients have a full-

Characterization and specificity of B-cell responses in lupus induced by Mycobacterium bovis in NOD/Lt mice

A single dose of pasteurized Mycobacterium bovis administered intravenously to prediabetic non-obese diabetic (NOD) mice prevented the onset of type 1 diabetes but precipitated a systemic 'autoimmune rheumatic disease' (ARD) similar to systemic lupus erythematosus. This syndrome was characterized by haemolytic anaemia, anti-dsDNA and anti-Smith antigen (Sm) antinuclear autoantibodies, increased severity of sialadenitis and glomerular immune complex deposition. Here, we examine the specificity of the autoantibody responses in M. bovis-treated NOD mice. Large amounts of antibody were detected to the Sm/ribonucleoprotein (RNP) complex, of which the 28 000 MW polypeptide appeared to be immunodominant. The IgG subclass involved in the anti-Sm response was primarily IgG2a. Antibodies against dsDNA were also detected, but the subclass of this response was mixed, with IgG2a and IgG2b being present in equal amounts. Together, these findings argue against a role for immune deviation towards T helper type 2 (Th2) responses in pathogenesis of the disease. The anti-dsDNA and anti-Sm reactivities were not mediated by polyreactive antibodies since neither antigen could cross-compete plasma antibody binding to the other in competitive enzyme-linked immunosorbent assay. The role of polyclonal B-cell activation was examined by measuring total gamma-globulin as well as IgG reactive with other nuclear antigens including Ro60, Ro52 and La, which although not a major component of the autoantibody responses in these mice, did show small but significant increases following immunization with M. bovis. Thus polyclonal stimulation, while likely to be occurring, was not directly responsible for production of anti-Sm antibodies.

Suppression of collagen-induced arthritis by continuous administration of IL-4

The onset of collagen-induced arthritis in DBA/1 mice is accompanied by a predominantly Th1 response, characterized by production of the proinflammatory cytokines IFN-gamma and TNF-alpha, and a predominance of IgG2a anti-collagen Abs. This study has primarily addressed the effects of continuous administration of exogenous IL-4, a Th2 cytokine, on collagen-induced arthritis in terms of time of onset, clinical symptoms, and histologic changes compared with those in untreated controls. The contributions of Th1 and Th2 cell responses were studied by examining anti-CII IgG subclasses, serum IgE levels, and cytokine production by synovial membrane and lymph node cell cultures. Continuous exposure to IL-4 for 28 days significantly delayed the onset of arthritis from 19 to 37 days and suppressed clinical symptoms. Arthritis occurred approximately 13 to 24 days after treatment ceased. Thereafter, the severity and duration of clinical symptoms were similar to those in control animals, although both joint damage and inflammation at the histologic and cellular levels were less severe than those in untreated controls. During IL-4 treatment, anti-collagen Ab levels were reduced (most significantly those of the IgG2a subclass), histology scores were lower, and the most striking effect was a 1000-fold decrease in TNF-alpha secretion by synovial cells. No significant differences in IgE levels were found between controls and IL-4-treated mice. These data suggest that the anti-inflammatory properties of IL-4 are mediated in part by down-regulation of Th1 responses rather than up-regulation of Th2 responses.

Suppression of collagen-induced arthritis by continuous administration of IL-4

The onset of collagen-induced arthritis in DBA/1 mice is accompanied by a predominantly Th1 response, characterized by production of the proinflammatory cytokines IFN-gamma and TNF-alpha, and a predominance of IgG2a anti-collagen Abs. This study has primarily addressed the effects of continuous administration of exogenous IL-4, a Th2 cytokine, on collagen-induced arthritis in terms of time of onset, clinical symptoms, and histologic changes compared with those in untreated controls. The contributions of Th1 and Th2 cell responses were studied by examining anti-CII IgG subclasses, serum IgE levels, and cytokine production by synovial membrane and lymph node cell cultures. Continuous exposure to IL-4 for 28 days significantly delayed the onset of arthritis from 19 to 37 days and suppressed clinical symptoms. Arthritis occurred approximately 13 to 24 days after treatment ceased. Thereafter, the severity and duration of clinical symptoms were similar to those in control animals, although both joint damage and inflammation at the histologic and cellular levels were less severe than those in untreated controls. During IL-4 treatment, anti-collagen Ab levels were reduced (most significantly those of the IgG2a subclass), histology scores were lower, and the most striking effect was a 1000-fold decrease in TNF-alpha secretion by synovial cells. No significant differences in IgE levels were found between controls and IL-4-treated mice. These data suggest that the anti-inflammatory properties of IL-4 are mediated in part by down-regulation of Th1 responses rather than up-regulation of Th2 responses.

Placental and fetal cardiac laminin are targets for cross-reacting autoantibodies from mothers of children with congenital heart block

The association of congenital heart block (CHB) with maternal autoantibodies to the Ro and La ribonucleoprotein antigens may be due to cross-reactions between maternal anti-La antibodies and fetal cardiac specific antigens. One of the major components of cardiac myocytes, laminin, is accessible for binding by maternal autoantibodies and we have previously reported cross-reactivity of mouse laminin with anti-La antibodies affinity purified from the sera of patients with primary Sjögren's syndrome. Affinity purified anti-La antibodies from ten women who had at some time given birth to a child with CHB were examined for cross-reactivity with human placental laminin, which shares structural similarities with cardiac laminin. All ten anti-La antibodies bound to the surface of cryosections of normal full term placental trophoblasts. Binding could be inhibited by pre-incubation of antibodies with either La or placental laminin. Eight anti-La antibodies also reacted with placental laminin by ELISA and La inhibited up to 82% of binding to laminin while laminin inhibited up to 85% of binding to La in a dose dependent manner. Eight anti-La antibodies also bound to the surface of fetal cardiac myocytes at 10.3 weeks of gestation and five showed lower levels of reactivity with the surface of fetal cardiac myocytes at 16.5 weeks of gestation. None showed any surface staining of normal adult heart. These data confirm the cross-reactivity of anti-La antibodies with laminin and may support a placental role in preventing the majority of potentially pathogenic antibodies from reaching the fetal circulation.

The expression of human endogenous retrovirus-3 in fetal cardiac tissue and antibodies in congenital heart block

Endogenous retrovirus-3 (ERV-3) is an endogenous retrovirus encoding an open reading frame for an envelope protein expressed in placenta. In this study we also found high levels of expression in fetal heart, with peak expression occurring between 11 and 17 weeks of gestation. Antibodies to a peptide corresponding to a predicted epitope of ERV-3 were studied by ELISA in sera from 32 healthy women, 47 women during pregnancy, 19 post-partum, 34 with Sjogren's syndrome (SS), 28 with systemic lupus erythematosus (SLE) and 48 mothers of babies with congenital heart block (CHB). Elevated levels of antibodies to ERV-3 were found in normal pregnancy and in patients with SS or SLE. Compared with normal sera the highest levels occurred in mothers of CHB babies (P < 0.001). Antibodies from sera from three CHB mothers bound to recombinant transmembrane protein of ERV-3 on immunoblots, and to sections of fetal cardiac tissue and placenta. This study has shown evidence of autoimmunization to ERV-3 during pregnancy, with particularly high levels of antibodies in mothers of CHB babies. The expression of ERV-3 in fetal heart and the presence of antibodies in maternal sera suggest a possible role in the pathogenesis of CHB.

Anti-La (SS-B) but not anti-Ro52 (SS-A) antibodies cross-react with laminin--a role in the pathogenesis of congenital heart block?

Cross-reactions between maternally derived autoantibodies and fetal cardiac antigens have been postulated to play a role in the pathogenesis of congenital heart block (CHB). We have explored the cross-reactivity of autoantibodies to the small ribonuclear autoantigens, La/SS-B and Ro/SS-A, with laminin, the major component of cardiac sarcolemmal membrane using affinity-purified antibodies from patients with Sjögren's syndrome (SS). Anti-La antibodies purified from eight of 10 patients cross-reacted significantly with mouse laminin by ELISA. In contrast, purified antibodies to Ro52 from the same 10 patients showed little or no binding to laminin. Laminin inhibited up to 70% binding of anti-La antibodies to La antigen, and La inhibited up to 65% binding of anti-La antibodies to laminin. The cross-reaction was further examined on cryosections of 10 human fetal hearts aged from 8.7 to 14.9 weeks of gestation, two normal adult hearts, and one pathological adult heart with a diagnosis of dilated cardiomyopathy. Anti-Ro52 antibodies did not bind to the surface of cardiac cells. However, anti-La antibodies from seven of 10 patients tested bound to the surface of fetal myocytes from hearts aged 9.4 to 14.9 weeks of gestation, and also to the myocytes from the pathological adult heart but not to normal adult hearts. Preincubation with La antigen abolished the binding of anti-La antibodies to the surface of adult heart myocytes with dilated cardiomyopathy, and pre-incubation with mouse laminin could partially block this binding. These results suggest that molecular mimicry between laminin and La, but not Ro52, may act as a target for specific maternal autoantibodies, and contribute to the pathogenesis of CHB at a critical stage during fetal cardiac development.

Mycobacteria precipitate an SLE-like syndrome in diabetes-prone NOD mice

Non-obese diabetic (NOD) mice spontaneously develop organ-specific autoimmunity and are widely used as a model for diabetes. Aged NOD mice also exhibit some features of non-organ-specific autoimmune rheumatic disease such as anti-nuclear antibodies and late-onset haemolytic anaemia. Here, we report that a single dose of 2.6 x 10(7) heat-killed bacillus Calmette-Guérin (BCG) i.v. in 8-week-old NOD mice prevented diabetes but precipitated a syndrome similar to systemic lupus erythematosus (SLE). Treated mice developed haemolytic anaemia, anti-DNA and anti-Sm anti-nuclear autoantibodies and an increased severity of sialadenitis. Perivascular lymphocytic infiltration in the kidneys and glomerular immune complex deposition were also found. The action of BCG appeared to be mediated by an adjuvant-like activity as treated mice showed a substantial increase in reticuloendothelial cell function and enhanced antigen presentation capacity.

Cross-reactive maternal autoantibodies and congenital heart block

Epitopes with linear sequences recognized by anti-La autoantibodies from seven mothers of children with congenital heart block were recently defined. Eight of these epitopes share sequence identity with three other proteins in addition to the original autoantigen, La. The three proteins are human cardiac myosin beta heavy chain, laminin B1 chain and the M6 protein of Streptococcus pyogenes. Affinity purified anti-La antibodies from a further three mothers bound to the La antigen and also to human cardiac myosin and mouse laminin. Affinity purified antibodies from three mothers of healthy children bound to the La antigen but showed minimal binding to either human cardiac myosin or mouse laminin. Cardiac myosin inhibited the binding of CHB-related anti-La antibodies to both La and myosin. These data support a role for maternal autoantibodies crossing the placenta and recognizing foetal cardiac antigens accessible at a critical developmental stage during gestation. We suggest that this would lead to complement fixation, inflammation and the subsequent pathology associated with congenital heart block.

Epitope mapping

The immune response to extrinsic and intrinsic antigens involves specific antigen receptors on T and B cells. The precise antigenic determinants, or epitopes, recognized by these receptors are discrete sequences within the native antigen. The ability to identify and manufacture key epitopes in the immune response has important implications for disease diagnosis and immunointervention. Consequently, increasingly sophisticated technologies are being applied to epitope mapping. This report from a recent workshop gives a balanced view of progress to date and the challenges ahead.

Ro and La antigens and maternal anti-La idiotype on the surface of myocardial fibres in congenital heart block

Congenital complete heart block (CCHB) is a rare but potentially fatal disease of infants born to mothers with autoimmune disease where maternal autoantibodies to Ro (SS-A) are thought to cross the placenta and damage fetal cardiac tissue. We have adopted a novel approach to demonstrate the localization and specificity of maternal autoantibodies deposited in fetal heart. We raised an anti-idiotype against maternal anti-La antibodies, which reacted strongly with the surface immunoglobulin on the myocardial fibres from a CCHB heart but not a control fetal heart of the same age. Maternal immunoglobulin eluted from the CCHB heart reacted with La (SS-B) by ELISA. Using monoclonal and affinity-purified antibodies to La and affinity-purified anti-Ro antibodies, both antigens were identified on the surface of the fibres of the affected heart. Surface co-expression of immunoglobulin, complement and Class II antigen, consistent with a local immune response, was also found. This is the first definitive demonstration of Ro and La antigens and specific maternal anti-La antibody and idiotype on the surface of myocardial fibres in CCHB. It suggests that induction of Ro and La antigens on the surface of myocardial fibres during fetal development may be critical in the localization of the specific autoantibodies and subsequent evolution of congenital complete heart block.

The response to Epstein-Barr virus infection in Sjögren's syndrome

To assess the response to Epstein-Barr virus (EBV) infection in patients with primary Sjögren's syndrome (SS), the frequency of detection of EBV DNA was studied in salivary gland biopsies and the antibody and idiotypic response to the virus was compared with healthy controls and infectious mononucleosis (IM). Viral DNA, detected by in-situ hybridization, was found in biopsies from two out of 12 patients with SS and six out of 10 controls. IgG, IgA and IgM antibodies to the virus, measured by ELISA using synthetic peptides (early antigen and EBNA-1) and a cloned fusion protein (EBNA-1), were normal in sera from 20 patients with SS, whereas infectious mononucleosis patients showed an increase in IgM antibodies to EBNA-1 and IgG antibodies to early antigen. One similarity between infectious mononucleosis and Sjögren's syndrome was a significant increase in the germline heavy chain idiotype G6 in both diseases, suggesting activation of similar B-cell subsets. It is possible that this is due to EBV, though the low frequency of EBV DNA in biopsies and the normal levels of EBV antibodies in SS does not lend any evidence that the virus itself is the causative agent.

Autoantibody synthesis in salivary glands of Sjögren's syndrome patients

We investigated the possibility that autoantibodies are locally synthesized and secreted within salivary glands of patients with Sjögren's syndrome by measuring specific autoantibody as a proportion of the total immunoglobulin present both in serum and saliva. Of 25 patients studied, 21 showed salivary enrichment of IgA anti-La, in three cases IgA anti-La being detected in saliva when IgG anti-La was negative (by ELISA) in serum. Twenty-four showed enrichment of salivary rheumatoid factors and IgA and/or IgM carrying the 17-109 idiotype, a marker of kappa IIIb light chains. These data suggest that autoantibodies, especially of the IgA class, are synthesized primarily in salivary gland and that they can be detected in saliva before they become apparent in the peripheral circulation. The subsequent deposition of these antibodies within salivary glands may be a contributory factor to the pathogenesis of Sjögren's syndrome.

Anti-idiotypic induced suppression of Sjögren's syndrome associated anti-La autoantibody secretion in vitro

Peripheral blood mononuclear cells from patients with Sjögren's Syndrome spontaneously secrete autoantibodies to the La antigen when cultured in vitro. This paper reports that specific IgG autoantibody production in vitro is suppressed by pre-treatment of CD8+ enriched T cells with rabbit polyclonal antibodies to idiotypes borne by circulating autologous anti-La antibodies. Treatment of this T cell subpopulation with anti-idiotypes specific for circulating anti-La antibodies from other patients or for anti-DNA antibodies was without effect on anti-La antibody production. Similarly anti-La anti-idiotypes had no effect on the production of autoantibodies to other ribonucleoprotein antigens such as nRNP/Sm. These data show that CD8+ T cells are the main targets for anti-idiotypic control in vitro. We suggest that the relative deficit of these cells, plus a surfeit of CD4+ T cells at the site of the pathological lesion within the salivary gland permits localized production of autoantibodies. Thus, dysregulation of the idiotypic network could contribute to the pathogenesis of Sjögren's Syndrome.

Co-existent anti-La antibodies and rheumatoid factors bear distinct idiotypic markers

This study describes the distribution of isotypes and idiotypes of two autoantibody populations, anti-La and rheumatoid factor, which co-exist in both the sera and saliva of patients with primary Sjögren's syndrome. The two autoantibodies are distinguished not only by their antigenic specificity but also by the nature of their idiotypic markers. IgA anti-La antibodies bearing restricted idiotypes are specifically enriched in saliva compared to serum suggesting their local synthesis. In contrast, rheumatoid factors bear cross-reactive idiotypes and may arise as a direct consequence of the secondary immune response.

Purification of human autoantibodies from cross-linked antigen immunosorbents

A method is described whereby autoantibodies to the Sjögren's syndrome antigen La (SS-B) can be purified from re-usable immunosorbent columns constructed from covalently linked human autoantibodies to which the antigen is cross-linked. Previous attempts to link the antigen directly to CNBr-Sepharose beads resulted in loss of biological activity and thus each purification of antibody required fresh batches of antigen. The present technique is a significant improvement since the cross-linked immunosorbents prepared from a single batch of antigen can be re-used several times over a 6 month period. Furthermore F(ab')2 fragments of anti-La antibodies can be purified from pepsin-digested serum samples. These antibodies react in ELISA, Western blot and immunofluorescence in an identical way to serum and murine monoclonal anti-La antibodies and show no reaction with the Ro antigen. However, being of human origin the affinity-purified anti-La antibodies have the advantages of bearing the same idiotypes and reacting with the same antigenic epitopes as naturally occurring serum autoantibodies.

Anti-C1q affinity isolated circulating immune complexes correlate with extra-articular rheumatoid disease

Circulating immune complexes (CIC) were isolated from sera of 35 patients with rheumatoid arthritis (RA) by a two-step method using 2% polyethylene glycol precipitation and anti-C1q affinity chromatography. By this method CIC were exclusive to 19 patients with vasculitis, nodules, or Sjögren's syndrome. Levels of CIC did not correlate with the severity of synovitis but reflected the extent of extra-articular disease. Furthermore, in four patients with persistent severe synovitis observed over a period of 4 to 16 months, the levels of CIC paralleled changes in extra-articular disease. Despite such additional evidence, whether the relationship between CIC and tissue injury is causative or consequential remains unresolved.

Idiotypes on antibodies to the La (SS-B) antigen are restricted and associated with the antigen binding site

Rabbit anti-idiotypic antibodies were prepared against affinity purified autoantibodies to the ribonucleoprotein La (SS-B) antigen from the sera of three unrelated patients. Each anti-idiotype recognized private idiotypes expressed only on the immunizing anti-La antibody. In each case they were conformationally dependent and related to the antigen binding site. This demonstration of immunodominant private idiotypes on human autoantibodies to ribonucleoproteins is in direct contrast to the cross-reactive idiotypes described on rheumatoid factors and autoantibodies to DNA. We discuss the possibility that anti-La antibodies, unlike anti-DNA, arise as a result of autoantigenic stimulation.

In vivo fate of 125I SS-B (La) injected into mice

The fate of the radiolabelled soluble cellular antigen SS-B (La) was compared with that of other 125I-labelled proteins of known molecular weight (MW) and electrostatic charge, following i.v. injection into BALB/c mice. The plasma half-life of 125I-SS-B was 3 min, while that of 125I-bovine serum albumin (similar MW and electrostatic charge) was 270 min. 125I-heat-aggregated IgG (MW greater than 1 x 10(6)) and 125I-7S human IgG (MW 168,000) had plasma half-lives of 40 min and greater than 300 min, respectively. Liver and kidney showed preferential uptake of 125I-SS-B, followed by a rapid decrease in radioactivity. During this time low MW, trichloroacetic acid (TCA) soluble, material appeared in urine. This suggests a specific uptake mechanism followed by a catabolic phase. These studies demonstrate that normal mice remove 125I-SS-B rapidly from the circulation and then degrade it. This rapid antigen elimination may protect against the induction of potentially harmful autoantibody responses.

The effect of IgG aggregate size and concentration on reactivity in immune complex assays

The reactivity of heat-aggregated IgG of known size, in the Raji cell assay, the C1q binding assay and the C1q solid phase radioimmunoassay as a function of concentration, has been investigated. Marked differences were found in the way that the three assays behave when the IgG concentration and aggregate size are varied. These findings indicate the pitfalls in attempting to express the results of immune complex assays performed on biological fluids in terms of equivalent concentrations of aggregated IgG.

Interpretation of the Raji cell assay in sera containing anti-nuclear antibodies and immune complexes

The Raji cell assay is regarded as a test for the detection and quantitation of immune complexes. It is frequently positive in sera from patients with SLE. We have demonstrated a relationship between Raji cell binding and antibodies to DNA and soluble cellular antigens. In five sera containing high titres of antibodies of known single specificity, most of the Raji cell binding occurred in the 7S IgG fraction where the majority of anti-nuclear antibody was also found. When each of these sera was incubated with its specific antigen, Raji cell binding increased. Subsequent fractionation showed that this binding was in the high molecular weight fraction (greater than 200,000 daltons) and that Raji cell binding and antibody activity were abolished in the 7S fraction. These data confirm that Raji cell bind immune complexes but also indicate that 7S anti-nuclear antibodies may interact directly with Raji cells by an unknown mechanism. Therefore, in sera of patients with anti-nuclear antibodies, binding to Raji cells does not necessarily imply the presence of immune complexes alone.

Nuclei of cultured synovial fibroblasts studied by immunofluorescence and microdensitometry

Two techniques which might be expected to detect alterations in DNA have been used in a comparison of cultured non-rheumatoid and rheumatoid synovial fibroblasts. Microdensitometry showed no alteration in staining affinity for methyl green when two pairs of cultures were compared. There was, however, a minor difference in the predominant staining pattern to a fluorescein conjugated anti-IgM serum when five rheumatoid cultures were compared with five non-rheumatoid cultures after exposure to a serum containing anti-nuclear antibodies.

The fractionation of isolated liver cells from normal and carcinogen treated rats

Suspensions of isolated cells were obtained from livers of normal rats and rats treated with the hepatocarcinogen N,N-dimethyl-4-aminoazobenzene. Differential centrifugation of dispersed cells yielded a large parenchymal cell fraction and a small non-parencymal cell fraction. By means of rate sedimentation through different concnetrations of Ficoll, parenchymal cells were separated into cells with fast, intermediate and slow rates of sedimentation. Periods of sedimentation were brief and centrifugal forces low in order to retain the best possible state of preservation of cells. DNA, RNA and protein contents, acid phosphatase activity, cell size and nucleocytoplasmic ratios of parenchymal cells sedimenting at fast, intermediate and slow rates were measured. Cell fractions from normal livers had properties suggesting that faster sedimenting cells were derived from the centre and middle of the lobule whereas slowly sedimenting cells were periportal; however, much of the periportal cell population remained in a residue of undissociated tissue. Compared with normal cells, carcinogen treated cells appeared to fractionate according to different physical and chemical criteria and could not be related to their origin within the liver lobule. They were smaller, slower sedimenting, lower in protein and RNA content and acid phosphatase activity. The tissue residue contained abnromal histological structures.