Table 1.

Many types of human CD21lo B cells identified in disease and healthy states

Disease/tissueDefining phenotypeIg isotypeSHMIn vitro responsesDisease correlatesTFsAdditional phenotypic markers
CVID CD21lo Mostly IgM Low (∼naïve) ↓ proliferation
↑ Ig secretion 
• Serum IFNγ, IFNγ-induced soluble factors (CXCL10)
• cTfh1 cells 
↑ SOX4 TBX21 (T-bet) TOX ↑ CD19 IgM CD95 CXCR3 CD22CD32 (FCGR2B) CD72 TNFRSF13B (TACI) ITGAX (CD11c) ITGB7 (CD49d)
↓ CD38 CD24 CD27 CXCR4CXCR5CCR7CD62L 
Tissue-based MBC FCRL4+CD27- Mostly IgG, IgA Similar to MBC ↓ proliferation
↑ Ig secretion 
NA ↑ RUNX1 SOX5 ↑ CD20 CXCR3 CD32 (FcRγIIB) CD80 CD86 CD11c CD95 RANKL CD40 CCR1 CCR5 CCR6
↓ CD21 CD31 CXCR4 CXCR5 CCR7 
HIV CD21loCD27 “Tissue-like/exhausted MBC” IgG (G1, G3), IgA, some IgM Intermediate between naïve and MBC ↓ proliferation
↓ Ig secretion 
• IFNγ-induced gene signature ↑ T-bet, RUNX3 SOX5 TOX1 ↑ FCRL4 CD72 CXCR3 CCR6 CD11c CD85j CD22 LAIR-1 TACI BCMA CD95
↓ CXCR4 CXCR5 CD62L CCR7 BAFF-R 
Malaria CD21loCD27lo
Atypical MBC 
IgG > IgM Similar to MBC
Derived from MBC 
↓ proliferation
↓ Ig secretion 
• Serum IFNγ ↑ T-bet
↓BACH 
↑ FCRL5 CXCR3 CD11c CD85j CD19 CD20 CD22 FcγRIIb
↓ CD27 CXCR4 CXCR5 CD62L CCR7 
HCV CD21lo ​ Similar to MBC ↓ proliferation ​ ↑ SOX5 ZEB2
↓BACH2 
↑ CD11c FCRL4 CD19 CD20 CD22 CD72 CD95 
cHBV infection (including HBsAg-specific B cells) CD21lo CD27± Mostly unswitched ND ↓ Ca2+ flux, BCR signaling
↓ Ig secretion
↓ cytokine production
↑ apoptosis 
• Low HBV-specific IgG ↑ T-bet ↑ CD11c CXCR3 FCRL5 CD22 BTLA PD1 FcγRIIb
↓ CXCR5 CD80 
SLE •IgDCD27 (DN)
•Activated B cells
• CD19hiCD20hi
• CD11c+
• CD24CD20hi 
IgG > naïve
< MBC 
↑ autoAb secretion
↑ apoptosis 
• Disease severity, serum autoAbs, renal involvement, lupus nephritis ↑ T-bet ZEB2 BCL6 IRF4 Blimp1± XBP1 • DN2: CXCR5neg CD19hi CD21lo CD11c+
• CD11c+: ↑ CD19 CD20, CD32 BAFF-R CD95 IL21R FCRL5 CD72 CXCR3 CCR9 SYK CD79B TLR9 CD80 CD86; TACIint,
↓ CXCR4 CXCR5 CCR7
• CD24CD20hi: CD21lo CD11hi CD95hi CD27± SLAMF7hi; ↑ PDCD1 FCRL3 ILIRB1 ILIRB2 
Other AI (RA, SjS, GVHD, T1D) CD21lo IgM, IgG, IgA Low (T1D) ↓ proliferation • Disease severity, autoAbs ↑ T-bet SOX4 SOX5 TOX ZEB2 ↑ CD19 CD22 CD86 CD58 CD11c  CD72 CD32 CD85j CD85d Fas RANKL (RA) ITGAX FCRL5
↓ CD40 BAFF-R IL-4R CCR7 CXCR4 CD44 CD62L
CD27int 
Trisomy 21 CD11c+ IgM, IgG, IgA Reduced Intact Ig secretion • Inflammatory cytokines (IL-6)
• cTfh1/17 cells
• Circulating PCs 
↑ T-bet ↑ CD86 CD95 CXCR3 CCR4
↓ CD21 CXCR5 CCR7 
Cancer scRNA-seq
CD21CD27 DN 
​ Low ↓ BCR response
↓ plasmablast generation but autoAb secretion 
• Poor outcomes
• Worse response to ICI blockade in some cancers 
TBX21 (T-bet) ZEB2 PRDM1IRF4XBP1TOXTOX2
IRF8 
FCRL4 FCRL5 CD32A CD32B CD72 ITGAX (CD11c) PDCD1 (PD-1) CD80CD86 SYK TLR7 TLR9 DUSP4 CD85j LAIR1
CR2 (CD21) CD27 CD38 
Vaccine recall CD21loCD27+
CD21loCD27CD11c+CD21lo 
​ ​ ↓ Ig secretion • Vaccine-specific IgG
• cTfh1 cells 
↑ T-bet Blimp1
XBP1 
↑ CD85j CD11c FCRL5 Ki67 CD80 CD95 BCMA
↓ CD38 CXCR4 CXCR5 
HDs CD21loCD27- FCRL5: IgM, IgD
FCRL5+: IgM, IgG 
FCRL5: low
FCRL5+: increased but <MBCs 
↓ proliferation compared with naïve B, but FCRL5 > FCRL5+
↑ apoptosis 
​ FCRL5+:
TOX TOX2 TBX21 SOX5 BCL6
FOXP1 EGR1 
FCRL5-: CD32+ LAIR1+ PD1+ CCR7lo CD62Llo CXCR3hi CXCR4lo CXCR5lo BAFF-Rlo CD11cdim CD20++ CD86+ CD95+ TACIdim
FCRL5+: CD22hi CD32hi LAIR1hi PD1hi CCR7lo CD62Llo CXCR3hi CXCR4neg CXCR5neg BAFF-Rlo CD11chi CD20++ CD86hi CD95hi TACI+ 

CVID, common variable immune deficiency; MBCs, memory B cells; cTfh cell, circulating T follicular helper cell; SLE, systemic lupus erythematosus; RA: rheumatoid arthritis; SjS: Sjogren’s syndrome; HBV, hepatitis B virus; HBsAg, hepatitis B surface Ag; HCV, hepatitis C virus; T1D, type 1 diabetes.

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