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My first concern is that control and test dogs were not matched for disease characteristics at the start of treatment to avoid inadvertently biased groupings in small experimental cohorts, where extensive individual variations exist. Also, the authors’ evaluations were not blinded. For example, their Supplementary Videos1 seem to show greater human encouragement of treated compared with untreated dogs, so functional recovery may not be independently verifiable by viewers, especially the “striking improvement of motility” in the older dogs treated with mesoangioblasts.

Second, Fig. 5a of Sampaolesi et al.1 indicates that muscle strength declined, rather than was maintained as they imply, in the treated legs of all dystrophic dogs in which it was assessed. At later time points, strength was always less than at 5 months. Between the ages of 5 and 9 months, the averaged decline in strength of the two older treated dogs was ∼40% (∼0.079 to ∼0.048 kg-1 (%)), even more than the ∼25% decline in the control untreated dystrophic dog over the same period. Against this real ∼40% decline, purported improvements of ∼50% and ∼80% in treated leg-muscle strength relative to contralateral, untreated muscles of these two dogs (Fig. 5b of Sampaolesi et al.1) are misleading and probably explained by simultaneous declines in contralateral muscle strength of ∼60% and ∼67%, respectively.

Third, the dogs Valgus, Varus and Vaccin each received 5 × 107 mesoangioblasts per treatment, but these were infused into the aortic arch of Valgus and the left femoral artery of the others. At biopsy, ∼30–70% of fibres in sections of Valgus’ left sartorius and gastrocnemius muscles appeared to be dystrophin-positive (Fig. 4a of Sampaolesi et al.1), whereas the equivalent dystrophin-positive proportion of Varus’ left sartorius was ∼0–10%, and that of Vaccin’s left gastrocnemius was only 0–5%. Numbers of mesoangioblasts reaching the lower left leg from upper aortic infusion, after major systemic blood diversions2, should have been ∼10 times less than from direct femoral infusion. However, no assessed muscles of Valgus had fewer—and certainly not 10 times fewer—dystrophin-positive fibres than corresponding muscles of Varus or Vaccin. Moreover, some of the untreated muscles of Varus and Vaccin revealed up to 50% dystrophin-positive fibres (Fig. 4a of Sampaolesi et al.1).

Minimal mesoangioblast recirculation3 cannot easily explain these anomalies, whereas dystrophin-positive fibres in both treated and untreated muscles might represent false positives or revertants4, rather than evidence of mesoangioblast engraftment. Control biopsies from pre-treatment and untreated dystrophic dogs4 would have allowed these possibilities to be differentiated. Alternatively, if recirculation and engraftment is responsible for similar percentages of dystrophin-positive fibres in treated and ‘untreated’ tibialis cranialis muscles (Fig. 4a of Sampaolesi et al.1), then contrasting their strengths (Fig. 5b of Sampaolesi et al.1) is unfounded.

There is an indicator of benefit arising from this trial, although it is, perhaps, due to concurrent immunosuppression rather than to mesoangioblasts. Supplementary Fig. 7 of Sampaolesi et al.1 shows that levels of the muscle-breakdown marker serum creatine kinase decreased markedly soon after initiating immunosuppression (as previously seen in mdx mice5) and before injection of heterologous mesoangioblasts, whereas in the days immediately after mesoangioblast injection, creatine kinase levels varied randomly, decreasing substantially (>5,000 U) in three instances but increasing in five. This does not support the authors’ hypothesis that creatine kinase reductions demonstrate mesoangioblast reconstitution of muscle fibres.

Control dystrophic dogs lived, on average, 129 days longer than six out of ten treated dogs (123 days longer than those treated with autologous mesoangioblasts). Three of the four remaining treated dogs, described as “well” 400 days post natal, “rapidly lost walking ability” when immunosuppression ceased (257 days post natal for two of these).

Altogether, the evidence presented by Sampaolesi et al.1 does not convince me that the dogs benefited from mesoangioblast treatment; a rigorous demonstration correlating muscle function, dystrophin expression and mesoangioblast infusion, with adequate controls, would have been helpful in this regard. It is therefore premature to consider a clinical trial in humans as a justifiable extension of this study.