Background Since 1992, we have observed many neonates who developed systemic exanthema and thrombocytopenia in the first week of life. Many neonatologists have reported a similar condition in Japan, since our report. Although most of the patients have been carriers of Staphylococcus aureus, no conclusive evidence of involvement of S. aureus in this disease has yet been obtained.

Methods 20 newborn infants who exhibited typical clinical symptoms were examined. In order to investigate whether superantigenic exotoxins (SAGTs) from S. aureus are involved in the pathogenetic mechanism of this disease, we examined production of SAGTs by S. aureus isolates from the patients, and expression of T cell receptor (TCR) Vβ and CD45RO in T cells by flow cytometry. We also analyzed the DNA sequences of 16 cloned Vβ2-positive TCR-β chain genes prepared from 2 patients. Findings Although most of the patients recovered within 5 days after the onset of their disease without any active treatment, two preterm patients died due to mucosal damage. All patients examined showed colonization by methicillin-resistant S. aureus (MRSA) that produced toxic shock syndrome toxin-1(TSST-1). TCR Vβ2-positive T cells, reactive to TSST-1, were selectively expanded and found to express CD45RO. None of the TCR Vβ2 clones had the same junctional sequences.

Interpretation The polyclonal expansion of TCR Vβ2-positive T cells in patients colonized by TSST-1-producing MRSA suggests that the pathogenic microorganism of this neonatal exanthematous disease is S. aureus, mainly MRSA, and that activation of T cells by TSST-1 is responsible for its pathogenesis. Although it seems likely that the pathogenetic mechanism underlying this disease and toxic shock syndrome (TSS) are fundamentally same, the manifestations of this disease are very different from those of TSS. The particular immune response in neonates could be related to the occurrence of this disease.