[Study on the association of pathologic myopia with HLA-DQB1 gene]

OBJECTIVE

To investigate the association of pathologic myopia (PM) with HLA-DQB1 alleles in order to study the pathogenesis of PM.

METHODS

The genome DNA of 66 patients with PM was extracted from the blood. The second exon of the HLA-DQB1 gene was amplified by polymerase chain reaction (PCR), and individual PCR products were digested by allele specific restriction enzymes of Hae III, BssH II, Apa I, BsaH I, Hae II, Hpa II, Rsa I, Bsp 1286 I for typing. Genotype was determined by restriction fragment length polymorphism (RFLP) pattern. The frequency of 16 alleles of HLA-DQB1 was detected and compared with that of the healthy control.

RESULTS

The frequencies of HLA-DQB1 * 0301 and * 0303 were significantly higher in the patients with PM (P < 0.0001), while that of HLA-DQB1 * 0601 and * 0602 were significantly lower (P < 0.0001) than that of the control group.

CONCLUSION

HLA-DQB1 * 0301, * 0303 alleles are susceptible alleles, and possibly they are the pathogenic genes, * 0601, * 0602 are resistant alleles, and may be they possess the protective property.

Parkinsonism as unusual neurological complication in childhood systemic lupus erythematosus

Parkinsonism complicating systemic lupus erythematosus (SLE) is extremely rare. We report two girls with SLE who developed extrapyramidal parkinsonian features after an initial stormy course. One patient presented with generalized tonic clonic seizure and was then noted to have akinetic mutism and masked face. MRI brain revealed abnormal signals in bilateral basal ganglia and single photon emission computed tomography (SPECT) showed hypoperfusion in the same area. EEG background was slow and disorganized. Symptoms persisted despite high dose intravenous methylprednisolone and cyclophosphamide. Intravenous immunoglobulin (IVIG) was prescribed empirically and was followed by complete recovery. Both EEG and MRI brain were normal on follow-up. The second patient was found unconscious and then developed bradykinesia, mutism and shuffling gait. MRI and SPECT both detected abnormalities in basal ganglia. EEG was slow. Intravenous immunoglobulin was given after methylprednisolone and cyclophosphamide. This was followed by clinical improvement. The pathogenesis of basal ganglia injury in SLE, along with the management of cerebral lupus and the mechanisms of action of IVIG, are discussed.

Occlusal considerations in determining treatment prognosis

To function in occlusal harmony, the masticatory apparatus--composed of the teeth and its supporting structures, temporomandibular joints, and associated neuromusculoskeletal structures--must operate in an integrated and dynamic manner. Loss of integrated function, or of homeostasis in response to functional demand, may generate problems in occlusion. In health, adaptive changes occur with the teeth and periodontium in response to functional occlusal forces. With periodontal and endodontic disease, this adaptive capacity diminishes. The ability to foresee how these changes may influence dental treatment is important in the art of determining treatment prognosis.

Role of FliJ in flagellar protein export in Salmonella

We isolated and characterized spontaneous mutants with defects in the 147-amino-acid Salmonella protein FliJ, which is a cytoplasmic component of the type III flagellar export apparatus. These mutants, including ones with null mutations, have the ability to form swarms on motility agar plates after prolonged incubation at 30 degrees C; i.e., they display a leaky motile phenotype. One mutant, SJW277, which formed significantly bigger swarms than the others, encoded only the N-terminal 73 amino acids of FliJ, one-half of the protein. At 30 degrees C, overproduction of this mutant protein improved, to wild-type levels, both motility and the ability to export both rod/hook-type (FlgD; hook capping protein) and filament-type (FliC; flagellin) substrates. At 42 degrees C, however, export was inhibited, indicating that the mutant FliJ protein was temperature sensitive. Taking advantage of this, we performed temperature upshift experiments, which demonstrated that FliJ is directly required for the export of FliC. Co-overproduction of FliJ and either of two export substrates, FliE or FlgG, hindered their aggregation in the cytoplasm. We conclude that FliJ is a general component of the flagellar export apparatus and has a chaperone-like activity for both rod/hook-type and filament-type substrates.

Genetic epidemiology study of pathological myopia

OBJECTIVE

To explore the genetic epidemiology of pathological myopia (PM), including hereditary and genetic model.

METHODS

The simple segregation analysis was done by SEGRANB. The values of segregation ratio p and the proportion of sporadic cases x were estimated. The complex segregation analysis was performed using SAGE-REGD. The genetic model and gene frequency were estimated. The 62 pedigrees with PM were random samples from hospital patients.

RESULTS

By simple segregation analysis, the genetic pattern of N*N is autosomal recessive and the frequency of sporadic cases is approximately 65.72%. The genetic pattern of A*N may be autosomal recessive (but autosomal dominant cannot be excluded), the frequency of sporadic cases is approximately 35.14%. By complex segregation analysis, the genetic model of PM is autosomal recessive and the gene frequency is 0.147385.

CONCLUSION

PM is compatible with autosomal recessive inheritance (autosomal dominant not excluded ), the sporadic cases are existent and the gene frequency is 0.147385.

[Combined triple/multi-surgery of vitrectomy-lensectomy and posterior chamber intraocular lens implantation]

OBJECTIVE

To study the feasibility and safety of the combined triple/multi-surgery of vitrectomy-lensectomy and posterior chamber intraocular lens implantation.

METHODS

Triple/multi-surgery was performed on 32 eyes of 32 cases, including 29 cases of serious eye injuries of traumatic cataract, lens luxation, vitreous hemorrhage (some cases with intraocular foreign body, iris damage or iridodialysis), 1 case with lens nucleus dislocated into the vitreous cavity during the senile cataract surgery and 2 cases with intraocular lens luxated into the vitreous cavity postoperatively.

RESULTS

After follow up for 1 - 14 months (mean 5.6 months), 15 cases had the best corrected visual acuity (BCVA) >or= 0.5 (46.9%), 11 cases had BCVA of 0.1 - 0.4 (34.4%), and 6 cases had BCVA < 0.1 (18.8%). There was an average of 2.10 DC of astigmatism after surgery. Macular edema was found in 14 cases (43.8%).

CONCLUSIONS

The results suggest that the triple/multi-surgery be safe and feasible. The major causes affecting the postoperative BCVA are astigmatism and macular edema. Proper selection of indication and skillful manipulation are the key points of the combined surgery.

[Confocal scanning laser Doppler retinal flowmetry in normal eyes]

OBJECTIVE

To introduce a new noninvasive equipment to measure the blood flow of optic nerve head and retina.

METHODS

The scanning laser Doppler retinal flowmeter (Heidelberg Engineering, Heidelberg, Germany) was used to measure the blood flow of optic nerve head and retina of 82 eyes of 48 normal individuals.

RESULTS

The average blood flow volume, blood flow velocity and the erythrocyte flow velocity of the vessel in optic nerve head were 203.14 +/- 74.77, 6, 272.43 +/- 2,260.07, 13.14 +/- 2.58 respectively; they were 26.36 +/- 14.74, 467.99 +/- 270.15, 1.93 +/- 1.07 in lamina cribrosa; 22.90 +/- 11.32, 514.53 +/- 370.21, 1.69 +/- 1.12 in the temporal rim of papilla; 21.77 +/- 9.83, 493.21 +/- 290.29, 1.65 +/- 0.90 in nasal rim of papilla; 32.55 +/- 13.00, 558.63 +/- 293.45, 1.78 +/- 0.83 in the temporal juxtapapillary retina; 20.61 +/- 8.92, 358.64 +/- 212.66, 1.20 +/- 0.66 in the nasal juxtapapillary retina; 28.89 +/- 11.52, 497.97 +/- 278.70, 1.64 +/- 0.83 in the avascular area of temporal retina; 25.00 +/- 8.04, 455.87 +/- 203.65, 1.55 +/- 0.64 in the avascular area of nasal retina.

CONCLUSIONS

Confocal laser scanning Doppler retinal flowmetry, which is based on confocal laser scanning technology and Doppler flowmetry, is an effective clinical method to determine the blood flow in optic nerve head and retina.

Intracellular generation of MDA-LYS epitope in foam cells

Oxidative stress plays a central role in atherogenesis. Antioxidants, such as probucol, inhibit oxidation of LDL, retard secretion of interleukin-1, growth factors and chemoattractants, and thus inhibit progression of atherosclerosis. Other antioxidants with an ability to inhibit LDL oxidation, however, could not prevent progression of atherosclerosis. The inconsistency between antioxidant potencies indicated oxidative events might have occurred at locations other than LDL. MDA-lysine epitope (MDA-lys) is closely associated with atherogenesis and was recognized as marker for oxidation. We traced formation of MDA-lys during oxidation of LDL and formation of foam cells. The results indicated that thiobarbituric acid reactive substance (TBARS) was primarily present in lipid fraction of ox-LDL not associated with protein fraction after Cu2+ oxidation in vitro. Oxidized LDL did not increase significant immunoreactivity of MDA-lys epitope under our experimental conditions. Foam cells, however, showed the presence of MDA-lys epitope suggesting that intracellular oxidation events occurred to internalized lipids. The uptake of non-oxidatively modified LDL (acetylated LDL) was sufficient to generate MDA-lys epitope in foam cells, consistent with the hypothesis that atherosclerosis is associated with oxidative events in addition to LDL oxidation. We hypothesized that MDA-lys may be generated through intracellular lipid metabolism during the formation of foam cells.

Characterization and quantitation of extracellular collagen matrix in myocardium of pigs with spontaneously occurring hypertrophic cardiomyopathy

The extracellular matrix between cardiocytes has been suggested to play an important role in maintaining the structure and function of the heart. The purpose of this study was to elucidate the morphological changes in the collagen of the extracellular matrix (ECM) in the hearts of pigs with hypertrophic cardiomyopathy. Sixty pigs diagnosed with hypertrophic cardiomyopathy from 605 purebred Landrace pigs ages 6 to 9 months were used in this study. Morphologically, these pigs with hypertrophic cardiomyopathy had increased heart weight and heart-to-body weight ratio, thickening of the left ventricular (LV) and right ventricular (RV) free walls and septum, disorientation of cardiocytes, myocardial fibrosis, and intramural coronary arteriosclerosis. Similar observations have been described in our preliminary report (Cardiovasc Pathol 3:261, 1994). In the present study, we have modified the silver impregnation technique to stain paraffin-embedded sections to demonstrate three types of ECM. There were endomysial struts, perimysial weaves, and epimysial coils in the myocardium. The light microscopic findings of the struts, weaves, and coils were also confirmed by scanning electromicroscopic examination. The numbers of these fine structures were increased significantly in the pigs with hypertrophic cardiomyopathy. In addition, the amounts of collagen in the LVs, RVs, and septum (Sep) in pigs with hypertrophic cardiomyopathy (LV = 19.37+/-0.79, RV = 23.72+/-0.72, Sep = 20.38+/-0.94 microg/mg, n = 60) were significantly higher (p < 0.01) than that in similar areas of normal pigs (LV = 14.56+/-1.11, RV = 18.90+/-1.02, Sep = 14.99+/-1.33 microg/mg, n = 30, respectively). Our findings of an overall increase of collagen content suggested that the accumulation of collagen matrix might be another factor responsible for the diastolic dysfunction of hypertrophic cardiomyopathy. These results might also infer that the increased collagen matrix could contribute to the stiffness of the cardiac chambers, thereby markedly affecting systolic and diastolic function of the heart. These observations provide further support that the pig may be an animal model for human cardiovascular disease.

Generation of HLA-DP transgenic pigs for the study of xenotransplantation

The shortage of human organs has prompted scientists to seek xenogeneic sources of donors. To date, DAF, MCP, and CD59 transgenic pigs have been generated to inhibit hyperacute rejection. However, besides hyperacute rejection, acute and chronic rejection must also be considered in the use of porcine organs for xenotransplantation. The role of HLA-II in transgenic xeno-organ transplantation remains to be elucidated. By microinjecting 1655 embryos, we have generated one stillborn HLA-DR and two live HLA-DP transgenic pigs: P113-7 (male, carrying one copy of exogene) and P113-8 (female, carrying 2-3 copies of exogenes). The gene status of the live transgenic pigs was confirmed by PCR, Southern blot, and PCR product sequencing analysis. The expression of transgenes in these transgenic pigs were confirmed by RT-PCR analysis and immunohistochemical staining of frozen sections of ear tissue.

A study to assess the effect of dietary supplementation with soluble fibre (Minolest) on lipid levels in normal subjects with hypercholesterolaemia

Hypercholesterolaemia is one of the major risk factors in the development of coronary artery disease. In recent years, many nonprescription treatments have become available for cholesterol lowering. Minolest is a product that contains guar gum and psyllium as the principal active ingredients. We conducted a randomised, placebo-controlled, double-blind, parallel-group study to assess the efficacy of Minolest as a lipid-lowering agent. Secondary aims included assessment of the effect on blood pressure and obesity. We also looked at the acceptability of the product and side effects associated with its ingestion. After a 4-week run-in period, 83 subjects were randomised to receive placebo or Minolest (16.5 g/day) for 3 months. Seven subjects defaulted follow up, 5 in the placebo group and 2 in the active treatment group. In addition, 9 subjects (5 on active treatment and 4 on placebo) had total cholesterol fall into the optimal range (< 5.2 mmol/l) during the run-in phase and were removed from the study. At baseline in the active treatment group, total cholesterol was 6.1 (5.43 to 8.06) mmol/l, triglyceride 1.54 (0.56 to 4.19) mmol/l, HDL cholesterol 1.32 +/- 0.43 mmol/l and LDL cholesterol 4.1 (3.10 to 6.27) mmol/l. In the placebo group, total cholesterol was 5.84 (5.32 to 8.38) mmol/l, triglyceride 1.47 (0.69 to 11.0) mmol/l, HDL cholesterol 1.15 +/- 0.33 mmol/l and LDL cholesterol 3.87 (2.46 to 5.14) mmol/l. The differences in the baseline characteristics were not statistically significant except the LDL-cholesterol. Minolest produced a 3.24% (SD = 7.85%, P = 0.020) decrease in total cholesterol and 5.45% decrease in LDL cholesterol (SD = 10.25%, P = 0.0034) but no significant difference in serum triglyceride, weight, body mass index or blood pressure. This was not seen in the placebo group. The percentage fall in LDL cholesterol increased to 7.16% and 7.37% in subjects who consumed at least 50% and 70% of the treatment respectively. There were few side effects. The authors conclude that this product has a small impact on the lipid profile and may be useful only in subjects with mild hypercholesterolaemia and a low risk of coronary artery disease.

Delineation of a membrane-proximal beta-rich domain in the GABAA receptor by progressive deletions

The type A gamma-aminobutyric acid (GABAA) receptor plays a major inhibitory role in the central nervous system. Structural elucidation of the GABAA receptor has been impeded by the large size of the receptor. We present here the delineation of a minimal structural domain as the first step of dissecting the receptor structure. This was achieved through prediction-assisted progressive deletions: the prediction of a candidate structural domain rich in beta-strands with no close similarity to known structures was tested by deleting putative secondary structure elements from the ends of the proposed domain, as well as mutations within the terminal secondary structures. Such progressive deletions revealed the limits of an integral domain, spanning Cys180 to Met293 (numbering of human alpha1 subunit). Below these limits the intact domain structure, as indicated by its circular dichroism, collapses. Based on its putative position, this domain is provisionally designated the membrane-proximal beta-rich domain of GABAA receptor. The inclusion of sequences from the first two out of four previously suggested transmembrane segments and one of the two conserved Cys residues in this domain defines important constraints to the receptor structure.

FQHC (Federally Qualified Health Center): harnessing the incentives of cost reimbursement

Community health centers (CHCs) are federally supported primary care providers to the low-income and uninsured. The federally qualified health center (FQHC) legislation requires states to pay CHCs for Medicaid services on the basis of reasonable cost. The statute generated controversy, particularly in a time when, for most providers, cost-related reimbursement has given way to fixed payments and managed care. This article examines the impact of FQHC on revenue and utilization of CHCs, using data for 328 centers that were in continuous operation between 1989 (the year the legislation was enacted) and 1992, the first year of full implementation. During this period, the CHCs Medicaid revenue grew rapidly. FQHC is estimated to account for under one third of the total increase, while inflation and growth in utilization due to expanded Medicaid eligibility are estimated to account for the other two thirds. At the same time, the change to cost-related reimbursement had a significant increase in total service users and Medicaid recipients receiving care from CHCs. Although some expected that cost-reimbursement would lead to inflationary increase in utilization, this did not occur. There was no statistically significant relationship between the change in payment methodology and changes in encounters per user. The experience of FQHC indicates that, for safety net providers of primary care, cost-related reimbursement is not "inherently inflationary." Results of this study raise the question of whether payment within constraints, but bearing relationship to cost, is not an appropriate approach to developing primary care capitation rates for these providers--and assuring maintenance of the safety net for the uninsured.

Fragment of GABA(A) receptor containing key ligand-binding residues overexpressed in Escherichia coli

GABA(A) receptor plays a major role in inhibitory synaptic transmission in the central nervous system and is the target of drugs such as the benzodiazepine tranquilizers. The polymeric membrane protein nature of GABA(A) receptor has rendered structural elucidation of the receptor a formidable task, greatly hampering structure-based drug design. We report here the first expression in Escherichia coli of a fragment of GABA(A) receptor. This 131-residue fragment, spanning Cys166 to Leu296 of human GABAA receptor alpha1 subunit, contains residues previously suggested to be involved in benzodiazepine binding. The overexpressed non-fusion recombinant protein was purified to near homogeneity and characterized by circular dichroism (CD), which showed that the recombinant protein has well defined secondary structures where beta-strands are dominant. The stability of the secondary structures was demonstrated by CD spectra at high pH and elevated temperature. Excluding part of the sequences from the carboxyl terminal of the fragment resulted in dramatic changes in the secondary structures comparable to the effects caused by SDS denaturation. Our results therefore suggest that the 131-residue fragment harbors an integral structural domain of the receptor. The overexpression of the recombinant protein fragment thus opens the way to the biochemical and structural studies of a functionally important region of the receptor, and exemplifies an effective approach of expression and characterization that potentially may be extended to other members of the ligand gated channel receptor superfamily, to which the GABA(A) receptor belongs.

Alteration of endogenous antioxidant enzymes in naturally occurring hypertrophic cardiomyopathy

We have recently developed a porcine model with naturally occurring hypertrophic cardiomyopathy (HCM). Similar to humans, occluded intramural coronary artery and damaged mitochondria are frequently observed in these animals in which the disease is thought to be associated with the local ischemia of myocardium. In view of antioxidant functions involved in the ischemic injury, we measured the expression of endogenous antioxidant enzymes in the tissues with and without HCM. The results showed a significant increase of Cu,Zn-superoxide dismutase (SOD), but not Mn-SOD, and decrease of catalase (CAT) activities in the various areas of HCM hearts. It was demonstrated that SOD/CAT ratios in the HCM hearts were significantly higher than those in normals and were found to be dramatically correlated with the severity of cardiac hypertrophy. The altered SOD/CAT ratio was also consistent with increase in lipid damage. We hypothesize that the elevated SOD combined with an inadequate amount of H2O2 scavenging enzyme may lead HCM heart at oxidative stress risk. However, the pathogenic role of imbalanced antioxidant enzyme needs to be further explored.

[A preliminary study on the association between HLA-DPB1 gene and pathological myopia]

OBJECTIVE

To investigate the association between distribution of human leucocyte antigen class II DPB1 alleles and pathological myopia (PM) among Chinese.

METHODS

The second exons of the HLA-DPB1 genes of 40 patients with PM were amplified by polymerase chain reaction (PCR), individual PCR products were digested by allele specific restriction enzymes: Bsp 1296 I, Fok I, Dde I, BsaJ I, BssH II, Rsa I, Ava II and EcoN I. Genotype of each patient was determined by restriction fragment length polymorphism (RFLP) pattern, then each DPB1 allele frequency of PM was calculated and compared with healthy control.

RESULTS

The frequency of DPB1*0301 allele was significantly decreased in PM (Yates corrected chi 2 = 4.33, Fisher exacted P = 0.032 < 0.05) and no statistically significant difference was observed after P value was corrected by the number of compared alleles. The rate of DPB1*0501/0501 homozygote has a significant difference between PM and healthy controls (u = 2.27, P < 0.05).

CONCLUSIONS

No genetic susceptible or resistant allele exists in HLA-DPB1 gene of PM. The significant increase of DPB1*0501/0501 homozygote in PM may be a linkage information.

Intramural coronary artery disease in swine with naturally occurring hypertrophic cardiomyopathy

Intramural coronary artery disease (ICAD) has been reported in myocardium affected with hypertrophic cardiomyopathy (HCM), but has never been studied in detail with respect to the cell type or lipid infiltration involved in the wall-thickening. The lack of heart samples may be one of the rationales to hamper the progress in investigating this disease. Recently, the discovery of naturally occurring HCM in swine has provided an excellent opportunity for the study of ICAD because of the high prevalence of ICAD in this animal. The present study provides a detailed structure feature in the thickened arterial wall of ICAD by both histologic and electron microscopic means. Morphologically, the feature of ICAD is due primarily to the neointimal thickening. Smooth muscle cells (SMC) and extracellular matrix (collagen and elastic fibers) are the major components responsible for the thickened neointima. Fragmentation of the internal elastic membrane is associated with the migration and proliferation of SMC from the media to the intima. Therefore, pigs with HCM may be a potential animal model not only for the study of the mechanism by which SMC migrate and proliferate into intima, but also for the future investigation of interventions in coronary artery occlusion.

One-step HPLC purification procedure for porcine brain 90-kDa heat shock protein

The 90-kDa heat shock protein (HSP90) was purified from porcine brain by a novel single-step purification procedure using diethylaminoethyl high-performance liquid chromatography (HPLC). About 4.8 mg of HSP90 was isolated from 25 g wet wt porcine brain tissue. The purified protein possessed a single moiety on one- and two-dimensional sodium dodecyl sulfate-polyacrylamide gel electrophoresis followed by silver staining. Western blotting using monoclonal antibody prepared against human HSP90 confirmed its identity as HSP90. These results indicate that small-scale HPLC purification of HSP90 from porcine brain tissue can be readily accomplished, with high yield, using a convenient one-step purification method. The procedure described in this paper represents a significant improvement in current purification methods for the isolation of HSP90 from porcine brain.

Ameliorative effect of MK-801 on retinal ischemia

The efficacy of MK-801, an N-methyl-D-aspartate receptor antagonist, was evaluated in a rat model of retinal ischemia induced by elevated intraocular pressure. Intraperitoneal injection of MK-801 at 0, 1, 3 and 10 mg/kg was given immediately after reperfusion. At 7 days after reperfusion, the inner retinal thickness, as measured from histologic sections of the retinas, of the 10 mg/kg treated group showed significant beneficial effect, while the other doses had no significant effect. Retinal ganglion cell counts on flat preparations of the retinas showed a beneficial dose dependent effect of MK-801 with the lowest dose showing no effect, 3 mg/kg showing marginal effects and 10 mg/kg showing significant effects. Intravitreal infusion of MK-801 during the ischemic period suppressed ischemia/reperfusion-induced internucleosomal DNA fragmentation measured at 18 hours after the insult as well as retinal tissue responses measured at 7 days. These findings suggested that the NMDA receptors may have an important role in ischemia-reperfusion insult as well as in mediating ischemia-induced apoptosis of retinal neurons. In addition, we demonstrated that pharmacological modulation of apoptotic cell death may affect the final tissue responses in vivo.

Early endosomes and a late endocytic compartment generate different peptide-class II MHC complexes via distinct processing mechanisms

Class II MHC Ag-processing compartments and mechanisms were compared for four antigenic epitopes from hen egg lysozyme (HEL) and RNase. T cell assays on subcellular fractions of Ag-pulsed macrophages detected the initial appearance of HEL-(48-61):I-Ak, HEL-(34-45):I-Ak, and RNase-(90-105):I-Ek complexes in a high density late endocytic compartment. In contrast, RNase-(42-56):I-Ak complexes never appeared in high density compartments, but were rapidly generated in low density endosomes. This early endosomal processing mechanism was 1) chloroquine inhibitable; 2) less sensitive than the late endocytic mechanism to 20 degrees C inhibition; 3) partially resistant to depletion of nascent class II MHC molecules with brefeldin A, suggesting some use of recycled class II MHC molecules, whereas the late endocytic processing mechanism was blocked by brefeldin A; and 4) involved in the processing of a DM-independent complex (RNase-(42-56):I-Ak). Thus, distinct processing compartments and mechanisms are identified for different epitopes even within a single Ag.

Early endosomes and a late endocytic compartment generate different peptide-class II MHC complexes via distinct processing mechanisms

Class II MHC Ag-processing compartments and mechanisms were compared for four antigenic epitopes from hen egg lysozyme (HEL) and RNase. T cell assays on subcellular fractions of Ag-pulsed macrophages detected the initial appearance of HEL-(48-61):I-Ak, HEL-(34-45):I-Ak, and RNase-(90-105):I-Ek complexes in a high density late endocytic compartment. In contrast, RNase-(42-56):I-Ak complexes never appeared in high density compartments, but were rapidly generated in low density endosomes. This early endosomal processing mechanism was 1) chloroquine inhibitable; 2) less sensitive than the late endocytic mechanism to 20 degrees C inhibition; 3) partially resistant to depletion of nascent class II MHC molecules with brefeldin A, suggesting some use of recycled class II MHC molecules, whereas the late endocytic processing mechanism was blocked by brefeldin A; and 4) involved in the processing of a DM-independent complex (RNase-(42-56):I-Ak). Thus, distinct processing compartments and mechanisms are identified for different epitopes even within a single Ag.