Clinically significant respiratory disease that requires mechanical ventilation support or 50% oxygen. Active, uncontrolled infection, or active systemic illness that requires ongoing vasopressor support or mechanical ventilation Requiring mechanical ventilation Subjects requiring vasopressor or mechanical ventilation. Pulmonary failure (requiring mechanical ventilation) not due to active LCH. Adequate respiratory function defined as not requiring supplemental oxygen or mechanical ventilation; oxygen saturation 90% or higher on room air RETREATMENT WITH MODIFIED T-CELLS EXCLUSION CRITERIA: Patient requiring supplemental oxygen or mechanical ventilation Requirement for mechanical ventilation within 7 days prior to Day 1, or sustained oxygen delivery for > 24 hours within 7 days prior to Day 1, or any oxygen requirement within 48 hours prior to Day 1 Adequate respiratory function defined as not requiring supplemental oxygen or mechanical ventilation; oxygen saturation 90% or higher on room air Adequate respiratory function defined as not requiring supplemental oxygen or mechanical ventilation; oxygen saturation 90% or higher on room air Adequate respiratory function defined as not requiring supplemental oxygen or mechanical ventilation; oxygen saturation 90% or higher on room air Subject does not require supplemental oxygen or mechanical ventilation, and oxygen saturation by pulse oximetry is 94% or higher on room air. Require mechanical ventilation or vasopressors for hemodynamic support at the time of randomization. Not requiring supplemental oxygen or mechanical ventilation; oxygen saturation 90% or higher on room air; no dyspnea at rest ELIGIBILITY CRITERIA FOR T-CELL PRODUCT INFUSION: Not requiring supplemental oxygen or mechanical ventilation, oxygen saturation 90% or higher on room air and no dyspnea at rest RETREATMENT WITH MODIFIED T CELLS: Patient requiring supplemental oxygen or mechanical ventilation Endotracheal intubation and mechanical ventilation at any FiO2 ELIGIBILITY CRITERIA TO UNDERGO OPTIONAL T CELL ABLATION\r\n* Research participant has >= 1% CAR T cells in the peripheral blood\r\n* Pulmonary: not requiring supplemental oxygen or mechanical ventilation, oxygen saturation 90% or higher on room air\r\n* Cardiovascular: not requiring pressor support, no symptomatic cardiac arrhythmias, no acute coronary syndrome, or uncontrolled hypertension\r\n* Renal Function: serum creatinine did NOT increase by more than 2.5 fold from baseline (at time of screening)\r\n* Liver Function: adequate liver function defined as total bilirubin =< 3.0 mg/dl \r\n* AST =< 5 x ULN, ALT =< 5 x ULN\r\n* Neurological: research participant without clinically significant encephalopathy/new focal deficits\r\n* Infectious diseases: no clinical evidence of uncontrolled active infectious process Not requiring supplemental oxygen or mechanical ventilation, oxygen saturation 90% or higher on room air (ELIGIBILITY CRITERIA AT TIME OF INFUSION OF GENETICALLY MODIFIED AUTOLOGOUS T CELLS): Not requiring supplemental oxygen or mechanical ventilation, oxygen saturation 90% or higher on room air Not requiring supplemental oxygen or mechanical ventilation, oxygen saturation 90% or higher on room air Pulmonary: not requiring supplemental oxygen or mechanical ventilation, oxygen saturation 90% or higher on room air Pulmonary criteria: not requiring supplemental oxygen or mechanical ventilation, oxygen saturation 90% or higher on room air Require mechanical ventilation or vasopressors for hemodynamic support at the time of enrollment. Clinically significant respiratory disease that requires mechanical ventilation support or 50% oxygen. Patients on mechanical ventilation. Not requiring supplemental oxygen or mechanical ventilation, oxygen saturation 90% or higher on room air Mechanical ventilation, renal replacement therapy, admitted in intensive care until at time of enrollment Has been on mechanical ventilation for > 28 days Currently on invasive mechanical ventilation or noninvasive positive pressure ventilation (CPAP or bilevel positive airway pressure) or requiring > 2LPM supplemental oxygen therapy to maintain O2 saturation > 90% due to hypoxemia Oxygenation index (OI) > 13, but < 37, for two consecutive blood gases which should be separated by at least one hour within 48 hours of the initiation of mechanical ventilation. Respiratory failure requiring mechanical ventilation or non-invasive ventilation Requiring mechanical ventilation Requires mechanical ventilation or is hemodynamically unstable at the time of randomization Respiratory failure requiring greater than 6 Lpm supplemental oxygen (O2) by nasal cannula or mechanical ventilation within the past six weeks