Lymphopenia and increasing viral load in the first 10 days of severe acute respiratory syndrome (SARS) suggested immune evasion by SARS-coronavirus (CoV). In this study, we focused on dendritic cells (DCs) which play important roles in linking the innate and adaptive immunity. SARS-CoV was shown to infect both immature and mature human monocyte-derived DCs by electron microscopy and immunofluorescence. The detection of negative strands of SARS-CoV RNA in DCs suggested viral replication. However, no increase in viral RNA was ob- IntroductionCoronaviruses (CoVs) comprise a large family of RNA viruses that infect a broad range of vertebrates, from mammalian to avian species. 1 Prior to the emergence of severe acute respiratory syndrome (SARS) in 2002 to 2003, human CoVs were known to be associated mainly with relatively mild upper respiratory diseases such as the common cold. The novel SARS-CoV, however, caused severe, rapidly progressive atypical pneumonia with fever, myalgia, and diarrhea. 2,3 The detection of virus in stool and urine in addition to the respiratory tract of patients with SARS further suggested that SARS is a systemic disease. 4,5 At autopsy, white pulp atrophy was observed in the spleen, and there was lymphoid depletion in lymph nodes. [6][7][8] Together with lymphopenia and increasing viral load in the first 10 days of disease, 3,4,6 these clinical features strongly suggest an evasion of the immune system by SARS-CoV. As with other viral infections, such as measles, this lymphoid depletion may have pathogenic significance.Dendritic cells (DCs) are antigen-presenting cells that play key roles in linking innate and adaptive immunity. 9-11 Immature DCs reside in the respiratory tract for immune surveillance, and they respond dynamically to local tissue inflammation in the airways and the distal lung. 12,13 They express a wide range of receptors, including c-type lectins 14,15 and toll-like receptors, 16,17 for the recognition of conserved pathogen patterns. Dendritic cells signal the presence of danger to cells of the adaptive immune response and modulate their responses via the secretion of proinflammatory and/or antiviral cytokines. 18 In particular, DCs secrete cytokines to polarize T-helper (Th) cells toward the Th1 or Th2 subsets. 10 The migration of DCs from tissues to lymph nodes is essential for antigen presentation and triggering of adaptive immune responses. The trafficking of DCs is regulated by chemokines that can be classified as homeostatic (constitutively expressed) or inflammatory (induced/augmented) according to their immune fuctions. [19][20][21] Acute respiratory viruses commonly induce inflammatory chemokines, such as macrophage inflammatory protein 1␣ (MIP-1␣), regulated upon activation, normal T cell expressed and secreted (RANTES), interferon-inducible protein of 10 kDa (IP-10), and monocyte chemotactic protein 1 (MCP-1), in local tissues. 21 Dendritic cells are also a major source of these chemokines. 20 On the basis of the function of DCs in immune surveillance, priming, a...

Human metapneumovirus (HMPV) is a newly discovered pathogen thought to be associated with respiratory disease. We report the results of a study of 587 children hospitalized with respiratory infection over a 13-month period. HMPV was detected in the nasopharyngeal aspirates from 32 (5.5%) children by reverse transcription-polymerase chain reaction. HMPV infection was associated with clinical diagnoses of pneumonia (36%), asthma exacerbation (23%), or acute bronchiolitis (10%). When compared to those with respiratory syncytial virus infection, children with HMPV infection were older, and wheezing was more likely to represent asthma exacerbation rather than acute bronchiolitis. HMPV viral activity peaked during the spring-summer period in Hong Kong. Phylogenetically, all HMPV virus strains from Hong Kong belonged to one of the two genetic lineages previously described. HMPV contributed to 441.6 hospital admissions per 100,000 population <6 years of age.