The development of conventional T and B lymphocytes is fine and strictly tuned¹. The phenotypical differences between T and B cells are usually thought to be clear, which can be readily discerned through detecting cell surface markers such as CD19, CD20, CD3, CD4, CD8, BCR and TCR. However, accumulating evidence indicates that the boundary between T and B cells could be ambiguous under both pathological and physiological circumstances.

T cell/B cell mixed phenotypic lymphocytes, such as CD3⁺CD20^+2,3 and CD3⁺CD19⁺ cells^4,5 have been observed in different disease contexts^3,6,7 and these biphenotypic cells are believed to play functional roles in multiple diseases, such as multiple sclerosis⁸, HIV infection⁹, rheumatoid arthritis¹⁰, ovarian cancer¹¹ and type I diabetes⁴. However, their presence and function in physiological conditions remain poorly understood. And Despite of being intensively observed, the origin of these biphenotypic lymphocytes is controversial^2,8,11-15.

In this study, we reported that a subset of lymphocytes endogenously expressing both T- and B-cell lineage markers (CD3, CD19, TCR-β, IgM, etc.) was identified in mice by the methods of multi-parameter flow cytometry and single-cell RNA sequencing and (Figure 1A and 1B). These CD3⁺CD19⁺ biphenotypic lymphocytes distributed widely in mouse bone marrow, lymph nodes, spleen, and peripheral blood and showed an origin of pro/pre B cells in pseudo-time analysis (Figure 1C). Furthermore, we found that these CD3⁺CD19⁺cells can be activated through both the TCR and BCR signaling pathways, and they actively participated in both cellular (Figure 1D) and humoral immune responses (Figure 1E) elicited by a candidate SARS-CoV-2 DNA vaccine. Interestingly, compared to conventional T cells (CD3⁺CD19^- cells), CD3⁺CD19⁺ cells secreted a higher level of IL-2 but a lower level of TNF-α upon antigen-specific stimulation (Figure 1D).

Figure 1. Functional T cell/B cell mixed phenotypic lymphocytes (CD3⁺CD19⁺)  in mice.

In addition to mice, we also identified CD3⁺CD19⁺ lymphocytes in freshly isolated human peripheral blood mononuclear cells. Notably, the human CD3⁺CD19⁺ lymphocyte subset exhibited comparable functionality to its murine counterpart, albeit with a lower frequency in humans than in mice.

Building upon these findings, our findings offered a fresh perspective on the complexity of the immune system, and highlighted that the T/B mixed phenotypic lymphocytes might played unrecognized roles in adaptive immunity. Further investigations are warranted to elucidate the precise mechanisms governing the development and the functionality of these biphenotypic lymphocytes, as well as their potential contribution to immune responses in different physiological and pathological contexts.

Reference

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