NEPHROLOGY – TRANSLATIONAL SECTION

A novel link: in children, cow milk processing maybe causative of idiopathic membranous nephropathy

Michael Haase • Anja Haase-Fielitz •

Peter R. Mertens

Received: 9 September 2011 / Accepted: 21 November 2011 / Published online: 20 December 2011

� Springer Science+Business Media, B.V. 2011

Summary of key findings

A causative pathway for ‘idiopathic’ membranous

nephropathy (MN) in children has been unraveled by

Debiec et al. [1], linking immunization to cationic

bovine albumin with antibody generation and subepi-

thelial immune complex depositions. The work per-

formed reads like a mystery story with unexpected

revelations, well suited to serve as exemplary for

translational research and the integrities that one may

encounter. The highlighted article by Debiec et al. [1]

describes a mechanism of membranous nephropathy

(MN) triggered by circulating cationic bovine serum

albumin (cBSA). In four children (aged \ 3 years)

diagnosed with MN, cBSA serum levels were associ-

ated with high titer of anti-cBSA autoantibodies in

serum samples. Epitope testing with peptide libraries

defined a cBSA subdomain (aa 147–161) as crucial for

antibody generation. This domain is a substrate of

trypsin acting in the gut and however may potentially

evade cleavage due to food processing. Cationic BSA

is predominantly co-localized with IgG4 in immune

deposits of the anionic outer glomerular capillary wall

and was eluted from it. Contrary to the majority of

adults with MN, no M-type phospholipase A2 receptor

(PLA2R) autoantibodies were detected in the affected

four children. The data set the stage for further

investigations and provide the rationale for therapeutic

approaches.

Review of the field

Idiopathic MN is the most common diagnosis for

glomerular diseases associated with nephrotic syn-

drome in adults—a condition related to excessively

high rates of thromboembolic and infectious compli-

cations [2]. Some studies indicate that with hypalbu-

minemia of less than 20 g/l the risk of developing

thrombosis exceeds 60% [3, 4], for affected patients

overt signs of nephrotic syndrome compromise quality

of life. To date, reliable biomarker(s) for the detection

of MN besides characteristic findings of subepithelial

and subendothelial immunoglobulin depositions in

kidney biopsies are not established. One of the urging

questions relates to therapeutic options in affected

patients. Good advice is not easy to provide, given that

the course largely depends on spontaneous remis-

sion(s) that are observed in approximately one-third of

all patients. One-third will develop partial remissions

and the remainder will eventually present with a pro-

gressive course, most often reflected by profound

proteinuria. To early-on decide in which group patients

belong to/guide decisions on the start of immunosup-

pressive medication is one of the mainstays; in this

M. Haase � A. Haase-Fielitz � P. R. Mertens (&)

Department of Nephrology and Hypertension, Diabetes

and Endocrinology, Otto von Guericke University

Magdeburg, Leipziger Str. 44,

39120 Magdeburg, Germany

e-mail: peter.mertens@med.ovgu.de

123

Int Urol Nephrol (2012) 44:635–638

DOI 10.1007/s11255-011-0099-3

regard marker proteins like urine b2-microglobulin

may help [5].

What are the options when an unfavorable course is

predicted? Again, advice has different aspects and

most treatment options have major drawbacks: Cyclo-

sporine may be helpful, and however, drug withdrawal

often results in relapses; cyclophosphamide is a

treatment option with severe side effects and toxicities,

and rituximab is expensive, does not succeed in all

patients and may also be accompanied by serious side

effects [6–9]. In these patients, it would be desirable

identifying a non-invasive test and modifiable mech-

anism of disease. Most therapeutic approaches rather

base on unspecific use of immunosuppressants than on

avoidance of antigen exposure or specific amelioration

of disease triggers.

A specific intervention targeting membranous

nephropathy is yet missing. Thus, let us start all over

again and think about the evolution of the disease. Is

our picture about the disease complete? The short and

exact answer is: no, we are on the verge to understand

underlying pathomechanisms and thus our under-

standing is very limited.

Membranous nephropathy (MN) is one of the most

common etiologies of nephrotic syndrome in adults.

The prevalence contrasts with numbers in pediatric

populations, where it is rarely diagnosed. Specifically,

the worldwide incidence of MN in children, reported

from more than 40 studies, is considerably less

than 10% of that in adults, where approximately 1.2/

100,000 cases per year are diagnosed [10]. Results

from a 17-year database covering a 275,000 child

population with 159 patients with mandatory referral

due to nephrotic syndrome suggest that MN occurs in

1.9% of the cohort (3/159 children) [11].

Novel data suggest that up to 50% of children with

MN will develop chronic kidney disease and/or

hypertension after a 4-year follow-up [12]. A renal

biopsy is inevitable for definite diagnosis which is

based on alterations including subepithelial immune

complex deposits, thickened capillary walls and matrix

spikes [13]. The most common differential diagnosis

includes minimal change nephropathy with favorable

course. Often steroids are applied, and depending on

the resolution of nephrotic syndrome, performance of a

biopsy is initiated.

What are pertinent questions and where is the

forefront of scientific endeavors? Under intense

investigation are endogenous or exogenous antigens

that precipitate MN. Therefore, a major interest is to

define exact antigen targets.

Several endogenous and exogenous antigens lead-

ing to immune response with the formation of immu-

noglobulin deposits in MN are known. Phospholipase

A2 receptor has been identified as a target antigen in the

majority of adults with MN. Circulating autoantibodies

against PLA2R have been found in 70–80% of tested

patients [14]. In an independent validation cohort of

42 adults [15], (1) the absence of circulating PLA2R

autoantibodies at the time of kidney biopsy did not rule

out a diagnosis of PLA2R-related MN and (2) the

presence of PLA2R autoantibodies in the blood did

not necessarily translate into PLA2R deposits in the

glomerulus.

Neutral endopeptidase, a podocytic antigen that

cleaves biologically active peptides was identified as

the target antigen of antibodies deposited in the

subepithelial space of glomeruli in a fetus developing

antenatal MN [16]. However, the latter mechanism of

disease may only account for a minority of patients.

Novel findings

The novelty of findings is given through the causal link

between circulating cBSA in children with the

formation of highly specific IgG4 autoantibodies

against a modified cleavage site in cBSA and their

glomerular co-deposition. The decline of cBSA and

autoantibody levels during the remission of disease is

in accord with a direct causal link of the disease, as

already suggested by experimental models [17].

Discussion

Cationic BSA is present in cow’s milk and beef protein,

may escape the intestinal barrier in children or compro-

mised people and may lead to autoantibody forma-

tion. Its cationic nature allows binding to the anionic

glomerular capillary wall with resultant immune com-

plex formation as shown in experimental models [18,

19]. This possible environmental trigger underlies the

hypothesis tested in the study.

There may be criticism that not all experiments or

follow-up observations could be performed in all

affected children. Actually, a complete experimental

series (verification of I. anti-cBSA antibodies in serum

636 Int Urol Nephrol (2012) 44:635–638

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and their epitope mapping, II. circulating cBSA, III.

glomerular cBSA and antibodies directed against it

and IV. elution of such IgG4 antibodies from glomer-

ular deposits) combined with long-term follow-up of

proteinuria was reported for a single child.

Nonetheless, the mechanistic approach of the

present work [1] and previous animal experiments

[17, 20] lend strong support to the pathogenetic role of

cBSA in MN in children (Fig. 1). The administration

of cBSA in immune-competent rats induced MN [17,

20]. Therefore, the data presented in this work may

allow speculating that in some young children with-

drawal of cow’s milk or other beef products may

reverse MN, and moreover, in other children, dietary

proteins to be determined might be involved in the

pathogenesis of idiopathic MN.

However, some issues await further studies. The

location (in vivo vs. ex vivo) and the trigger of the

formation of cationic bovine serum albumin detected

in the affected patients remain ambiguous. A control

group of exclusively breast-fed children with MN and

investigation into cBSA or other pathological antigens

may be informative. Mothers of these four children

have not been investigated as potential carriers of the

genetic information for a modified BSA ingestion or as

potential donors of cBSA to their children. It remains to

be determined how the antigen/antibody complexes

cause glomerular injury. Finally, evidence for a disease

remission through avoidance of antigen exposure,

specifically withdrawal of cow’s milk or meat (prod-

ucts) in children, would be the proof of concept.

Even though some cases of ‘‘idiopathic’’ MN in

children may be reclassified as secondary MN with

cow’s milk-mediated autoimmunity, there are remain-

ing cases of MN still awaiting the discovery of specific

causative agents. Therefore, confirmation of study

findings in other case series is desirable.

In future studies, several specific and conceptional

questions need to be addressed, including the clinical

value of cBSA screening as a biomarker for prediction

of MN, identification of further modifiable mecha-

nisms involved in the pathogenesis of MN and

exploration of age- and pH-dependent gastrointestinal

passage of BSA. Are there antibodies to other dietary

proteins responsible for the induction of glomerular

immune deposits/MN and how long before clinical

onset of MN are cBSA (autoantibodies) detectable?

Although these shortcomings have to be addressed in

the near future, the methodological approach used in the

work by Debiec et al. [1] is exemplary and should be

translated to uncover immunological pathomechanisms

of other idiopathic autoimmune glomerulopathies.

Take home message

There is a novel antigen identified that may contribute

to autoantibody formation, namely cationic bovine

heat-processed milk/beef products

childhood membranous nephropathy

anionic glomerular capillary

wall

food intake exposure immunization

bovine serum

albumin (BSA)

protein modification

cow milk formula

Children aged

0-3 years

Anti-bovine serum albumin antibodies

(IgG4 >> IgG1)

Subepithelial immune deposits:

circulating cationic BSA +anti-BSA autoantibodies

pH: children: basic adults: neutral

no M-type PLA2R immune deposits

cBSA+Ab

in-situ complex formation

Fig. 1 Sequelae of events

leading to food-mediated

early childhood

membranous nephropathy

Int Urol Nephrol (2012) 44:635–638 637

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albumin. Intriguingly, autoimmune responses occur in

adults but not children.

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