Characterization and Activity of TIM-1 and IL-10-Reporter Expressing Regulatory B Cells

In addition to their role in humoral immunity, B cells can exhibit regulatory activity. Such B cells have been termed regulatory B cells (Bregs). Bregs have been shown to inhibit inflammatory immune responses in a variety of autoimmune, alloimmune, and infectious settings. Breg activity is frequently IL-10-dependent, although a number of other mechanisms have been identified. However, our understanding of Bregs has been hampered by their rarity, lack of a specific phenotypic marker, and poor insight into their induction and maintenance. A variety of B-cell subsets enriched for IL-10⁺ Bregs have been identified in multiple murine disease models that can adoptively transfer Breg activity. However, most of these B-cell subsets actually contain only a minority of all IL-10⁺ B cells. In contrast, TIM-1 identifies over 70% of IL-10-producing B cells, irrespective of other markers. Thus, TIM-1 can be considered a broad marker for IL-10-expressing Bregs. Moreover, TIM-1 signaling plays a direct role in both the maintenance and induction of Bregs under physiological conditions, in response to both TIM-1 ligation and to apoptotic cells. TIM-1 expression has also been reported on IL-10⁺ human B cells. Together, these findings suggest that TIM-1 may represent a novel therapeutic target for modulating the immune response and provide insight into the signals involved in the generation and induction of Bregs. Here, we provide the methods to analyze and purify the murine TIM-1⁺ B-cell subset for further in vitro and in vivo experiments. We also provide methods for in vitro analysis and in vivo tracking of Bregs using IL-10-reporter mice.