Lymphocytes in the lung: an often neglected cell. Numbers, characterization and compartmentalization

Area of Science:

• Immunology
• Pulmonary Medicine
• Cell Biology

Background:

• The lung's constant exposure to inhaled particles necessitates robust immune defenses.
• Understanding pulmonary immune reactions requires knowledge of lymphocyte localization.
• Lymphocytes, crucial for immune responses, are often under-characterized in respiratory tract studies.

Purpose of the Study:

• To outline the compartmentalization, number, and subset composition of lymphocytes within the lung.
• To describe the intrapulmonary migratory routes of lymphocytes.
• To explain the lung's integration into the common mucosal immune system.

Main Methods:

• Histotopographic analysis of lymphocyte distribution in lung tissues.
• Characterization of lymphocyte subsets in different lung compartments (epithelium, lamina propria, vascular bed, interstitium).
• Analysis of lymphocytes obtained via bronchoalveolar lavage.

Main Results:

• Lymphocytes are present in the bronchial epithelium and lamina propria with varying subset compositions.
• Bronchus-associated lymphoid tissue (BALT) is observed in some species and can be induced in humans.
• The lung vascular bed harbors numerous lymphocytes, and the interstitium contains approximately 10 x 10^9 lymphocytes in adults.
• Bronchoalveolar lavage yields primarily "memory lymphocytes".

Conclusions:

• Detailed knowledge of lung lymphocyte distribution is essential for understanding pulmonary immunity.
• The lung possesses unique features, including a large lymphocyte vascular pool and inducible BALT.
• Lymphocyte populations in the lung, particularly memory cells, play a critical role in mucosal immunity.