P160 High levels of CD19 expression is a characteristic feature of age-associated B cells in ageing and autoimmunity
Authors: Shah Kavina , Poon Vanessa , Taylor Samuel J , Klein Christian , Kollert Florian , Schuler Franz , Mathur Sanjana , Castelino Madhura , Sen Debajit , Cambridge Jo , Cragg Mark , Leandro Maria , Akbar Arne , Reddy Venkat 
Published: April 2024

Abstract

Background/Aims
Recent evidence implicates an important role for age-associated B cells (ABCs), defined as CD19+IgD-CD27-T-bet+ B cells, in healthy ageing, following viral infection and in active SLE. We know that co-receptors of the B cell receptor (BCR) such as CD19 and other activating receptors can influence B cell activation through distinct pathways. Therefore, we compared the expression of CD19 in B cell subpopulations including ABCs in ageing and SLE.

Methods
Peripheral blood was obtained from six young (&lt;45 years) healthy participants, seven elderly (&gt;65 years) healthy participants and eight patients with SLE (25-65 years). We performed flow cytometry on peripheral blood mononuclear cells to analyse the expression of extracellular and intracellular markers. We performed statistical analysis using GraphPad Prism v7.

Results
CD19hi B cell frequency was at least 10-fold higher in T-bet+ cells as a whole and particularly enriched in IgD-CD27-T-bet+, ABCs, in all groups, p &lt; 0.05. The geometric mean fluorescent intensity (gMFI, expression) of CD40 was significantly lower in IgD-CD27-T-bet+ subpopulations compared to IgD-CD27-T-bet- B cell subpopulations in all groups (Table). In all groups, in CD19+T-bet+ cells, compared to CD19+T-bet- cells, there was a significantly increased frequency of IFNγR+ cells, TLR4+ cells, TLR7+ cells and TLR9+ cells (p &lt; 0.05 for all) with remarkable difference noted in SLE patients for TLR7 and TLR9, p &lt; 0.01 (Table). In the switched memory IgD-CD27+ and ABCs, TLR7 expression was significantly higher (p &lt; 0.01) in SLE patients (mean 43.8% and 18.5% respectively) compared to elderly participants (mean 13.5% compared to 4.3% respectively).

Conclusion
High levels of expression of CD19 is a characteristic feature of ABCs and CD19hiT-bet+ cells predominantly belong to the IgD-CD27-T-bet+ ABC subpopulation. CD19hi IgD-CD27-T-bet+ cells also had significantly lower expression of CD40 and higher expression of IFNγR and TLR7 and TLR9, suggesting that CD19 plays a significant role in moderating activation of ABCs triggered through T-independent, IFNγR-TLR-T-bet pathway. Taken together with the significant role of CD19 in regulating signalling threshold of B cell activation, and its stable expression across different B cell developmental stages, these data provide compelling mechanistic rationale for targeting CD19 to deplete B cells, including potentially pathogenic age-associated, in autoimmune disease such as SLE.

Disclosure
K. Shah: None. V. Poon: None. S.J. Taylor: None. C. Klein: Corporate appointments; CK is employed by Roche. Shareholder/stock ownership; CK has patents and stock ownership at Roche. F. Kollert: Corporate appointments; FK is employed by Roche. Shareholder/stock ownership; FK has stock ownership at Roche. F. Schuler: Corporate appointments; FS is employed by Roche. Shareholder/stock ownership; FS has patents and stock ownership at Roche. S. Mathur: None. M. Castelino: None. D. Sen: None. J. Cambridge: None. M. Cragg: None. M. Leandro: None. A. Akbar: None. V. Reddy: None.