Of note, lymphopenia, as noticed in our patient, is the most common feature reported in HSCT patients with hMPV, accounting for 73% of patients in one series [7] . This illustrates that although innate immune responses are stimulated upon hMPV exposure, adaptive immunity also appears important to control hMPV. As for other paramyxoviruses, the matrix proteins are involved in the induction of proinflammatory and Th1 responses by dendritic cells and macrophages (i.e., production of interleukin-2 and interferon-γ) [11] . Inflammation may cause diffuse alveolar damage and hyaline membrane formation as shown by histopathology investigations [12] .