muscle specimens were also examined by electron microscopy , which was performed as described previously .
b - cell clonality was assessed in both patients ' specimens by pcr amplification using a set of biomed-2 assays ( invivoscribe technologies , san diego , ca ) .
loci for igh [ igha : fr1 ( variable region framework 1)-j ; ighb : fr2-j ; ighc : fr3-j ] and for igl ( v - j ) were targeted .
muscle specimens were also examined by electron microscopy , which was performed as described previously .
b - cell clonality was assessed in both patients ' specimens by pcr amplification using a set of biomed-2 assays ( invivoscribe technologies , san diego , ca ) .
loci for igh [ igha : fr1 ( variable region framework 1)-j ; ighb : fr2-j ; ighc : fr3-j ] and for igl ( v - j ) were targeted .
patient 1 , a 27-year - old woman , had a 7-year history of unilateral swelling of her left biceps muscle and hand as well as wrist pain and morning stiffness of her finger joints .
a contrast - enhanced mri of her left biceps showed diffuse gadolinium uptake , thickening of the muscle fascia , and muscle edema ( figure 1 ) . on neurologic examination she showed normal symmetrical muscle strength and normal fine motor skills .
laboratory examination of her blood serum showed elevated immunglobulin e , c - reactive protein , and antinuclear and anti - cyclic citrullinated peptide antibodies .
based on the international consensus criteria , biopsy of the left biceps gave a diagnosis of dm with accompanying b - cell - rich follicle - like structures . an initial high - dose ( 1 mg / kg / d ) prednisolone treatment over 4 weeks followed by a low - dose treatment ( 5 mg / d ) led to rapid improvement and persistent resolution of her symptoms over a 1-year period .
contrast - enhanced mri ( t1 ) revealed areas of hyperintensity with diffuse gadolinium uptake in the affected muscle .
patient 1 : left biceps ( c ) ; patient 2 : right deltoid and left triceps brachii muscles ( a , b ) .
patient 2 , a 17-year - old girl , was referred because of a 5-month history of painful reddish swellings of the left upper arm followed by similar symptoms in her right upper arm .
three weeks prior to referral she had arthralgia of the hands , knees , and feet , and morning stiffness . on neurologic examination
she had a normal serum ck and erythrocyte sedimentation rate and a positive antinuclear antibody titer .
mri of the upper arms revealed hyperintensity in the right deltoid and left triceps muscles ( figure 1 , a and b ) .
a biopsy of the left triceps showed a picture consistent with dm with accompanying b - cell - rich follicle - like structures .
she was treated with methylprednisolone pulse therapy ( 1,000 mg for 3 consecutive days ) and monthly iv immunoglobulin ( ivig ) and subsequently received 60 mg prednisone daily and methotrexate ( 17.5 mg ) weekly .
both ivig and prednisone were successfully tapered , and currently she receives subcutaneous methotrexate and hydroxychloroquine , which has led to sustained improvement of her symptoms .
except for some minor alterations of the skin there remained no signs of active disease .
muscle biopsies of both patients revealed extensive infiltration by inflammatory cells mainly in the perimysium and endomysium ( figure 2 , a 
mac ( c5b-9 ) deposition was mainly found on small capillaries and on the sarcolemma of single muscle fibers ( figure 2 g , arrowhead ) .
mhc class ii was expressed by the lymphoid cells and was found on numerous muscle fibers , predominantly in the perifascicular area ( figure 2i ) .
sarcolemmal mhc class i expression was detected on all muscle fibers with perifascicular enhancement ( figure 2j ) .
combined cytochrome c oxidase ( cox)/succinate dehydrogenase histochemistry revealed scattered cox - negative fibers as a sign of accompanying mitochondrial dysfunction ( figure 2k ) . cd123 dendritic cells ( figure 2l ) and cd138 plasma cells ( figure 2 m ) were found in close proximity to t- and b - cell areas . the lymphoid follicle - like structures ( lfls )
were mainly composed of cd45 leukocytes ( figure 2e ) with cd8 ( figure 2n ) and cd4 ( figure 2o ) t cells distributed around cd79 b cells ( figure 2p ) .
expression patterns of peripheral bcl-2 ( figure 2q ) , central bcl-6 ( figure 2r ) , and bob.1 ( figure 2s ) paralleled the specific lymphoid follicle - like b - cell pattern .
the ki67/mib-1 proliferation index was increased within the center of the follicle - like structures ( figure 2 t ) .
serial sections of muscle biopsy stained with hematoxylin & eosin revealed ectopic lymphoid follicle - like structures ( patient 1 : a and b ; patient 2 : c and d ) .
sections were stained with an antibody against cd45 leukocytes ( e ) to highlight the follicle - like inflammatory infiltrate .
c5b-9 was mainly found in walls of small capillaries and on the sarcolemma of single muscle fibers ( g , arrowhead ) .
electron microscopy revealed ultrastructural evidence of undulating tubules ( patient 1 : h , arrowheads , 50,000 ; patient 2 : not shown ) .
major histocompatibility complex ( mhc ) class ii was expressed by the lymphoid cells and was found on numerous muscle fibers , predominantly in the perifascicular area ( i ) .
combined cytochrome c oxidase ( cox)/succinate dehydrogenase staining revealed blue - stained fibers indicating reduced cox activity ( k ) .
the lymphoid follicle - like structures consisted of cd123 dendritic cells ( l ) , cd138 plasma cells ( m ) , and cd8 ( n ) and cd4 ( o ) t cells that were distributed around cd79a ( p ) b cells .
expression patterns of bcl-2 ( q ) , bcl-6 ( r ) , and bob.1 ( s ) illustrate lymphoid follicle - like structures with increased proliferative activity in the b - cell areas as indicated by ki67/mib-1 immunostaining ( t ) .
scale bars : a , d , e , f , k : 200 m ; b , c , g , s : 50 m ; i , j , l 
electron microscopy revealed undulating tubules in endothelial cells of both patients , which supported the light microscopic diagnosis of dm ( figure 2h , arrowheads ) . to exclude the presence of a neoplastic b - cell population as described for b - cell lymphomas within skeletal muscle , we performed a b - cell clonality analysis .
muscle biopsies of both patients revealed extensive infiltration by inflammatory cells mainly in the perimysium and endomysium ( figure 2 , a 
mac ( c5b-9 ) deposition was mainly found on small capillaries and on the sarcolemma of single muscle fibers ( figure 2 g , arrowhead ) .
mhc class ii was expressed by the lymphoid cells and was found on numerous muscle fibers , predominantly in the perifascicular area ( figure 2i ) .
sarcolemmal mhc class i expression was detected on all muscle fibers with perifascicular enhancement ( figure 2j ) .
combined cytochrome c oxidase ( cox)/succinate dehydrogenase histochemistry revealed scattered cox - negative fibers as a sign of accompanying mitochondrial dysfunction ( figure 2k ) . cd123 dendritic cells ( figure 2l ) and cd138 plasma cells ( figure 2 m ) were found in close proximity to t- and b - cell areas .
the lymphoid follicle - like structures ( lfls ) were mainly composed of cd45 leukocytes ( figure 2e ) with cd8 ( figure 2n ) and cd4 ( figure 2o ) t cells distributed around cd79 b cells ( figure 2p ) .
expression patterns of peripheral bcl-2 ( figure 2q ) , central bcl-6 ( figure 2r ) , and bob.1 ( figure 2s ) paralleled the specific lymphoid follicle - like b - cell pattern .
the ki67/mib-1 proliferation index was increased within the center of the follicle - like structures ( figure 2 t ) .
serial sections of muscle biopsy stained with hematoxylin & eosin revealed ectopic lymphoid follicle - like structures ( patient 1 : a and b ; patient 2 : c and d ) .
sections were stained with an antibody against cd45 leukocytes ( e ) to highlight the follicle - like inflammatory infiltrate .
c5b-9 was mainly found in walls of small capillaries and on the sarcolemma of single muscle fibers ( g , arrowhead ) .
electron microscopy revealed ultrastructural evidence of undulating tubules ( patient 1 : h , arrowheads , 50,000 ; patient 2 : not shown ) .
major histocompatibility complex ( mhc ) class ii was expressed by the lymphoid cells and was found on numerous muscle fibers , predominantly in the perifascicular area ( i ) .
combined cytochrome c oxidase ( cox)/succinate dehydrogenase staining revealed blue - stained fibers indicating reduced cox activity ( k ) .
the lymphoid follicle - like structures consisted of cd123 dendritic cells ( l ) , cd138 plasma cells ( m ) , and cd8 ( n ) and cd4 ( o ) t cells that were distributed around cd79a ( p ) b cells .
expression patterns of bcl-2 ( q ) , bcl-6 ( r ) , and bob.1 ( s ) illustrate lymphoid follicle - like structures with increased proliferative activity in the b - cell areas as indicated by ki67/mib-1 immunostaining ( t ) .
scale bars : a , d , e , f , k : 200 m ; b , c , g , s : 50 m ; i , j , l  r , t : 100 m .
electron microscopy revealed undulating tubules in endothelial cells of both patients , which supported the light microscopic diagnosis of dm ( figure 2h , arrowheads ) . to exclude the presence of a neoplastic b - cell population as described for b - cell lymphomas within skeletal muscle , we performed a b - cell clonality analysis .
since both patients initially presented with unilateral muscle swellings , no detectable muscle weakness , and lack of ck elevation , the diagnostic criteria of dm , according to the european neuromuscular centre , and jdm , according to the international consensus criteria , were not present .
however , the morphologic criteria to diagnose dm were perfectly fulfilled , including the presence of undulating tubules known to be detectable in early - stage dm , emphasizing the need for muscle biopsy in these patients .
muscle biopsy of both patients also showed characteristic lfls with defined t- and b - cell compartmentalization , which is a very rare finding in a very small subset of patients with jdm and  overlap syndrome . 
ectopic lymphoid follicles are a well - known histopathologic feature in other autoimmune diseases , including multiple sclerosis with meningeal inflammation , sjgren syndrome affecting salivary glands , and focal myositis .
nevertheless , it remains unclear which factors lead to formation of lfls in these diseases .
the lymphoid aggregates present in our muscle biopsies show a remarkable similarity to secondary lymphoid organs ( slos ) with distinct t- and b - cell compartmentalization .
expression of specific germinal center ( gc ) b - cell markers ( bob.1/bcl-6 ) in the central b - cell - rich areas suggests a gc - like reaction in dm , similarly seen in true slos .
it was previously reported that jdm patients with lymphoid follicle - like organization of the inflammatory infiltrate in muscle have a more severe and difficult - to - treat disease .
however , our patients , who slightly exceeded the age at onset of jdm , exhibited a rather mild clinical presentation and responded favorably to therapy .
brachio - cervical inflammatory myopathy , focal myositis , and  overlap syndrome  were discussed as potential differential diagnoses and ruled out based on histologic , clinical , and ultrastructural ( presence of undulating tubules ) features .
the outcome in patients with idiopathic inflammatory myopathy ( iim ) has been studied recently .
most patients ( 68% ) with iim ( including dm ) developed a polyphasic or chronic disease course .
there were no significant differences between the different myositis subtypes with regard to disease course , mortality , and quality of life .
we show that the presence of lfls is not necessarily evidence of an unfavorable prognosis in adult dm as opposed to reports in jdm . in conclusion
goebel designed the study concept , did the analysis of data , participated in data acquisition , and drafted and revised the manuscript .
schonenberg - meinema participated in the acquisition of data and drafted and revised the manuscript .
heppner is on the scientific advisory board for baxter ; holds a patent application for modulators of il-12 and/or il-23 for the prevention or treatment of alzheimer disease ; is the founding scientist of myosotis therapeutics ag ; and is a consultant for sanofi and novartis .
m. de visser is on the editorial board for neuromuscular disorders and clinical neurology and neurosurgery .
goebel is on the editorial advisory boards for neuromuscular disorders , muscle & nerve , neuropathology and applied neurobiology , brain pathology , and clinical neuropathology .
w. stenzel is on the editorial board for neuromuscular disorders and neuropathology and applied neurobiology .
 objective : to investigate the clinical and morphologic spectrum of early adult  onset dermatomyositis ( dm ) , an inflammatory disease that affects small vessels of the muscle and the skin.methods:histologic evaluation of frozen muscle samples was employed to visualize the cellular organization of ectopic lymphoid structures in muscle biopsies obtained from 2 patients diagnosed with dm . clinical presentation and morphologic 
 features , as well as treatment and follow - up , were assessed and documented . 
 electron microscopy was used to confirm the light microscopic diagnosis of dm . 
 clonality analysis of b - cell populations using pcr was performed.results:muscle biopsy of both patients fulfilled the morphologic european neuromuscular centre criteria of dm . 
 analyses of muscle biopsy samples revealed ectopic lymphoid follicle - like structures that showed a remarkable similarity to secondary lymphoid organs ( slos ) with distinct t- and b - cell compartmentalization . 
 our 2 patients exhibited an atypical and mild clinical presentation and responded favorably to therapy.conclusions:the clinical and histopathologic features of dm can be atypical , and the presence of slos is not inevitably linked to an unfavorable prognosis . 
METHODS
Histologic, enzyme histochemical, and immunohistochemical studies.
Electron microscopy.
B-cell clonality analysis.
RESULTS
Muscle morphology.
DISCUSSION
Supplementary Material
AUTHOR CONTRIBUTIONS
STUDY FUNDING
DISCLOSURE