The Impact of CD30 on Germinal Center B Cell Dynamics and the Expansion of IgG1-Switched B Cells

Background

CD30 (also known as TNFRSF8) is a member of the tumor necrosis factor receptor (TNF-R) superfamily. Initially identified as a marker of Hodgkin lymphoma, subsequent research revealed its expression on other lymphomas, such as diffuse large B-cell lymphoma and primary effusion lymphoma, as well as on the surface of activated B and T cells. Under physiological conditions, the number of CD30-expressing B cells is very low and typically restricted to either germinal center (GC) or non-GC B cell phenotypes located within or around the GC.

CD30 is activated via interaction with its ligand, CD30 ligand (CD30-L, also known as CD153), which is expressed by activated T cells and other cells. CD30 and CD30-L interact in trimeric structures, triggering the recruitment of TNF-R-associated factors (TRAFs) and the activation of various signaling pathways, including the NF-κB, MAPK, and JAK/STAT pathways. However, the specific role of CD30 in B cells remains elusive. While studies using transgenic mice revealed no significant B cell-related phenotype upon CD30 deletion, recent research suggests that CD30 signaling plays a key role in T-cell-dependent (TD) immune responses.

Research Motivation and Open Questions

Despite extensive studies on the expression of CD30 in B cells, its specific function in this cell type remains unclear. Particularly in certain immune disorders and viral infections, the number of CD30-expressing B cells increases significantly, but the underlying mechanisms and functional consequences remain unknown. To address this knowledge gap, this study created a conditional CD30-knockin mouse model to investigate the role of CD30 in B cells and its effects on germinal center responses and B cell differentiation.

Research Team and Publication Details

This study was conducted by a research team that includes Yan Wang, Ursula Rambold, and Petra Fiedler from the Helmholtz Center Munich, Germany. The paper was published online on October 17, 2024, in the journal Cellular & Molecular Immunology (DOI: 10.1038/s41423-024-01219-w).

Study Design and Experimental Workflow

1. Development of a Conditional CD30-Knockin Mouse Model

To explore the function of CD30 in B cells, the researchers created a conditional CD30-knockin mouse model. The detailed steps are as follows:

• Genetic Engineering: The mouse CD30 cDNA was inserted into the Rosa26 locus along with a loxP-flanked transcriptional/translational stop cassette upstream of the gene. A truncated form of the human HCD2 gene was added downstream of CD30 as a reporter gene, controlled by an internal ribosomal entry site (IRES).
• Mouse Breeding: The CD30stopfl/+ mice were crossed with CD19-Cre mice to generate CD30stopfl/+//CD19-Cre+/- mice (hereafter referred to as CD30//CD19-Cre mice), enabling specific expression of CD30 in B cells.
• Validation Experiments: Flow cytometry and Western blot analyses confirmed the increased expression of CD30 in the splenic B cells of CD30//CD19-Cre mice and validated the deletion of the stop cassette via HCD2 reporter gene expression.

2. Expansion of B, T, and Myeloid Cells in Aged Mice

The team observed that young CD30//CD19-Cre mice exhibited a normal B cell phenotype, but significant expansions of B cells, T cells, and myeloid cells occurred in aged mice (17–19 months). Key findings include:

• Increased Spleen Weight: In 68% of aged CD30//CD19-Cre mice, spleen weights exceeded 0.25 grams, compared to only 31.6% in controls.
• Cell Number Expansion: B, T, and myeloid cell numbers increased significantly in aged CD30//CD19-Cre mice, and CD30 expression levels correlated positively with spleen weight.
• Changes in B Cell Subsets: Higher numbers of B1b cells and plasmablasts were observed in aged CD30//CD19-Cre mice, while follicular B cells (FOBs) decreased.

3. Impact of CD30 Expression on Germinal Center Responses

To investigate how CD30 impacts germinal center dynamics, the team observed:

• Increased Spontaneous Germinal Centers (GCs): Aged CD30//CD19-Cre mice had significantly higher percentages of spontaneous GC B cells compared to controls.
• Expansion of IgG1-Switched B Cells: These mice also exhibited a greater proportion of IgG1-switched B cells, which displayed either a GC or memory-like phenotype.

4. Effects of CD30 Expression on T Cells and Myeloid Cells

Aged CD30//CD19-Cre mice exhibited an expansion of CD30-L-expressing senescence-associated T cells (SA-T cells) and follicular helper T cells (TFH cells). Reciprocal interactions between these T cells and CD30+ B cells stimulated further expansion of B and T cell populations.

5. Influence of CD30 on Immunization Responses

Using immunization studies, the team discovered that CD30 expression facilitates faster antigen-activated B cell entry into GCs and enhances plasma cell differentiation. CD30 signaling also promoted the expansion of IgG1-switched B cells, which displayed GC or memory-like B cell phenotypes.

Key Results and Conclusion

1. Role of CD30 Signaling in B Cells

This study provides the first evidence demonstrating the critical role of CD30 signaling in B cells. CD30 expression in aged mice led to the significant expansion of B, T, and myeloid cells and enhanced germinal center responses and IgG1-switched B cell formation.

2. Modulation of Immune Responses by CD30

CD30 signaling regulated the expression of CXCR4, which accelerated B cell entry into GCs and enhanced plasma cell differentiation. Moreover, CD30 signaling promoted the expansion of IgG1-switched B cells with GC or memory-like phenotypes.

3. Implications for Disease Mechanisms

The findings provide new insights into the pathological mechanisms in immune disorders (e.g., systemic lupus erythematosus and rheumatoid arthritis) characterized by an increased number of CD30+ B cells. Aberrant activation of CD30 signaling may contribute to pathological B cell expansion and immune dysregulation.

Study Highlights

• Innovative Mouse Model: The conditional CD30-knockin mouse model offers a powerful tool for studying the function of CD30 in B cells.
• Mechanistic Insights: The study demonstrates how CD30 signaling modulates germinal center dynamics, B cell differentiation, and plasma cell formation through CXCR4 regulation.
• Relevance to Disease: The findings provide a foundation for understanding the role of CD30 signaling in immune diseases and may inform future therapeutic strategies.

Significance and Broader Implications

This research not only advances the understanding of CD30 signaling in B cell function but also sheds light on its potential role in the pathology of immune disorders. By elucidating the mechanisms by which CD30 signaling drives B cell expansion and immune responses, this study offers a theoretical basis for developing targeted therapies for diseases associated with aberrant CD30+ B cell populations.