Early derivation of IgM memory cells and bone marrow plasmablasts

Abstract

IgM memory cells are recognized as an important component of B cell memory in mice and humans. Our studies of B cells elicited in response to ehrlichial infection identified a population of CD11c-positive IgM memory cells, and an IgM bone marrow antibody-secreting cell population. The origin of these cells was unknown, although an early T-independent spleen CD11c- and T-bet-positive IgM plasmablast population precedes both, suggesting a linear relationship. A majority of the IgM memory cells detected after day 30 post-infection, also T-bet-positive, had undergone somatic hypermutation, indicating they expressed activation-induced cytidine deaminase (AID). Therefore, to identify early AID-expressing precursor B cells, we infected an AID-regulated tamoxifen-inducible Cre-recombinase-EYFP reporter strain. Tamoxifen administration led to the labeling of both IgM memory cells and bone marrow ASCs on day 30 and later post-infection. High frequencies of labeled cells were identified on day 30 post-infection, following tamoxifen administration on day 10 post-infection, although IgM memory cells were marked when tamoxifen was administered as early as day 4 post-infection. Transcription of Aicda in the early plasmablasts was not detected in the absence of CD4 T cells, but occurred independently of TLR signaling. Unlike the IgM memory cells, the bone marrow IgM ASCs were elicited independent of T cell help. Moreover, Aicda was constitutively expressed in IgM memory cells, but not in bone marrow ASCs. These studies demonstrate that two distinct long-term IgM-positive B cell populations are generated early in response to infection, but are maintained via separate mechanisms.

T-Bet+ IgM Memory Cells Generate Multi-lineage Effector B Cells

“…In the (AID-Cre-ER T2 x Rosa26 eYFP) F 1 mice, all cells expressing AID at the time of tamoxifen administration irreversibly express eYFP ( Dogan et al, 2009 ). In our studies, few if any cells were labeled in uninfected (AID- Cre-ER T2 x Rosa26 eYFP) F 1 mice, indicating that the labeled cells were infection specific ( Papillion et al, 2017 ). Although our previous studies focused on CD11c + IgM memory cells, eYFP + B cells detected after tamoxifen administration were found to be more diverse.…”

“…Our previous studies demonstrated that CD11c + IgM memory cells elicited during E. muris infection undergo limited somatic mutation and showed that these cells could be irreversibly labeled in (activation-induced cytidine deaminase [AID]-Cre-ER T2 x Rosa26 enhanced yellow fluorescent protein [eYFP]) F 1 mice ( Papillion et al, 2017 ). In the (AID-Cre-ER T2 x Rosa26 eYFP) F 1 mice, all cells expressing AID at the time of tamoxifen administration irreversibly express eYFP ( Dogan et al, 2009 ).…”

“…Even though these pathogens are largely confined to host cells, humoral immunity plays an essential role in host defense ( Bitsaktsis et al, 2007 ; Li et al, 2001 ). Although we first identified IgM memory cells in our studies by their unique expression of CD11c, we later demonstrated that these cells also express T-bet as well as a number of cell surface markers characteristic of memory B cells, including CD73, PD- L2, and several integrins ( Papillion et al, 2017 ; Yates et al, 2013 ). A number of additional criteria were used to establish that the CD11c + T-bet + B cells identified in our infection model are IgM memory cells: (1) they persist for at least one year post-infection; (2) they are not actively dividing; (3) they do not spontaneously produce IgM or IgG; (4) they do not express markers characteristic of GC cells; and (5) depletion of the CD11c + B cells ablated secondary IgG responses to antigen, independent of CD11c-negative bone marrow plasma cells ( Yates et al, 2013 ).…”

T‐bet+ memory B cells: Generation, function, and fate

“…Using the Ehrlichia muris infection model, they identified the emergence of IgM + CD11c + splenic plasmablasts 45 and memory B cells 46 that were later recognized to express T-bet. 47 These memory cells are required to generate an IgG response to secondary challenge, 46 and bone marrow IgM + antibody-secreting cells, which may arise from IgM + Tbet + precursors, protect from fatal E. muris challenge. 48 49 These cells produced virus-specific IgG2 a following in vitro stimulation and were necessary for maintaining reduced viral titers.…”

“…35 Considering the requirement for IFNγ or IL-21 signaling for optimal T-bet + B cell development, 73 these studies favor a germinal center-dependent origin for the subset. However, extrafollicular development of T-bet-expressing plasmablasts has been reported in E. muris-infected mice, 45,47 suggesting the potential for germinal center-independent generation under certain conditions.…”