A 12-year-old girl presents with a 3-week history of non-productive cough, dyspnea, and low-grade fevers. Her symptoms started shortly after her family adopted a pet cockatiel. On examination, she is tachypneic with fine crackles at the lung bases. A high-resolution computed tomography scan shows diffuse ground-glass opacities and ill-defined centrilobular nodules. Which of the following findings on bronchoalveolar lavage (BAL) would be most consistent with the suspected diagnosis?