Mouse study maps how porcine circovirus type 2d reshapes immune cells

Porcine circovirus type 2, a small nonenveloped DNA virus roughly 17 nanometers across, is one of the most economically damaging pathogens of the global swine industry, and the PCV2d genotype has displaced PCV2b in several regions thanks to its enhanced pathogenicity and transmission efficiency. A new study published in Animal Diseases on 10 July 2026 set out to map how this dominant genotype manipulates the mammalian immune system, using C57BL/6 mice as a tractable experimental model. The researchers, led by Yonghui Qiu and Jiawei Liu with colleagues including Jue Liu, rescued a replication-competent PCV2d strain in the laboratory and then tracked its effects on lymphocyte activation, thymic T-cell survival, memory T-cell differentiation, and regulatory T-cell expansion over four weeks of infection.

The team began by constructing an infectious clone from the full-length PCV2d genome of strain PCV2-LD398-2020, flanked by two EcoRI restriction sites so the circular viral DNA could be released by digestion and self-ligation. After transfection into PK-15 cells and serial passaging, quantitative PCR confirmed viral genomic DNA at passages 3, 8, and 13, and Western blotting detected a specific Cap protein band of approximately 28 kilodaltons, indicating stable replication. Immunofluorescence assays showed that the Cap protein localized predominantly to the nucleus with a minor cytoplasmic presence. By passage 13 the viral titer had reached 10^6.33 TCID50 per milliliter, confirming successful rescue of a replication-competent PCV2d strain.

Sixty six-week-old female C57BL/6 mice were then divided evenly into an infected group and a mock-infected control group. Infected animals received an intraperitoneal injection of 0.2 milliliters of PCV2d at 10^6.33 TCID50 per milliliter, while controls received the same volume of culture medium. At 4, 7, 14, 21, and 28 days post-infection, the researchers collected blood and harvested the liver, spleen, lung, thymus, and lymph nodes for nucleic acid extraction, flow cytometry, and fixation for pathological analysis. Body weight gain did not differ significantly between the groups, indicating that PCV2d infection did not induce growth retardation under these experimental conditions.

Despite the absence of visible growth effects, the virus established itself efficiently in immune organs. PCV2d-specific antibodies were first detectable at 7 days post-infection and peaked at 28 days, confirming a robust humoral response. Quantitative PCR revealed significantly higher viral loads in the spleen and lungs than in the thymus and lymph nodes, with the spleen carrying the greatest burden and thus serving as the primary site of in vivo replication. At 28 days, histopathological examination documented mild hepatocyte vacuolar degeneration with bile stasis, reduced splenic white pulp cells alongside red pulp reticular cell necrosis, alveolar wall rupture with epithelial cell shedding in the lungs, and focal thymic atrophy with a blurred corticomedullary boundary. Immunohistochemistry detected abundant PCV2d antigen in the liver, spleen, lungs, and thymus, with the highest levels in the spleen and lungs, and antigen-positive cells were often found in areas of tissue damage, such as alveolar epithelial cells in the lungs, directly linking viral presence to injury.

The most detailed findings came from flow cytometric analysis of splenic lymphocyte activation based on CD69 expression. The pattern was both time-dependent and subset-specific. CD8+ T cells, which kill virus-infected cells, were activated throughout the infection period, with stronger activation at 7 to 14 days post-infection than at 21 to 28 days. γδ T cells, innate-like lymphocytes that respond rapidly to infection, were activated only in the early phase, from 7 to 14 days. CD4+ T cells, which provide helper signals for B cells and CD8+ T cells, and natural killer cells were inhibited at 7 days post-infection but became activated in the late stage, from 14 to 28 days. B cells, responsible for antibody production, remained activated throughout the entire infection, a pattern consistent with the gradual rise in PCV2d-specific antibodies measured by ELISA. Serum levels of IL-2, TNF-α, IFN-γ, and IL-10 measured by ELISA were consistent with these lymphocyte activation data.

The thymus, the organ where T cells mature, showed striking subset-specific damage. Using Annexin V and viability dye staining, the researchers distinguished early from late apoptosis among thymic T-cell populations. CD8+ T cells underwent significant apoptosis throughout infection, with early apoptosis at both 7 and 28 days post-infection and late apoptosis on days 7, 14, and 21, suggesting that this cytotoxic subset is directly damaged by the virus. CD4+CD8+ double-positive thymocytes, an intermediate maturation stage, primarily underwent both early and late apoptosis in the early phase. CD4+ T cells primarily showed early apoptosis during the late stage, from 21 to 28 days, and late apoptosis during the early stage, from 7 to 14 days, reflecting dynamic shifts in the mechanisms of cell death. CD4−CD8− double-negative thymocytes, an immature population, exhibited significant early apoptosis during the late stage, from 14 to 21 days, without concurrent late apoptosis, indicating that immature T cells are particularly vulnerable to PCV2d-induced cell death.

Population dynamics in the thymus mirrored this destruction. The numbers of CD8+, CD4+, and CD4−CD8− T-cell populations initially expanded during the early phase before declining during the late phase, from 21 to 28 days post-infection. In contrast, the CD4+CD8+ double-positive population decreased as early as 4 days post-infection, suggesting rapid impairment of thymic function. The authors interpret the early expansion of immature CD4−CD8− cells as a compensatory proliferative response to infection, while the subsequent reduction in mature CD4+ and CD8+ cells appears to stem directly from apoptosis. The unique sensitivity of CD8+ T cells to both early and late apoptosis is notable because it implies that PCV2d specifically targets the very subset responsible for viral clearance.

In the spleen, PCV2d infection reshaped the memory landscape of both major T-cell lineages. Based on CD44 and CD62L expression, CD4+ and CD8+ T cells were classified into naive, central memory, and effector memory subsets. The percentage of naive CD4+ T cells fell significantly in infected mice, while the percentages of central memory and effector memory CD4+ T cells rose, indicating that infection drives the differentiation of naive cells into memory subsets. Among CD8+ T cells, the naive percentage was significantly lower in infected mice and the central memory percentage increased. The effector memory CD8+ percentage rose at 14 days post-infection, coinciding with early CD8+ T-cell activation, and then fell at 21 days, a transient increase the authors suggest may represent an initial cytotoxic response. In parallel, PD-1, an immune checkpoint molecule whose upregulation is a hallmark of T-cell exhaustion, was significantly upregulated on the central memory and effector memory subsets of both CD4+ and CD8+ T cells during infection, which the authors suggest may contribute to viral persistence.

Regulatory T cells, defined as CD4+CD25+Foxp3+ cells that suppress excessive immune responses to maintain tolerance, also responded to infection. Their numbers in the spleen began to increase at 14 days post-infection, rose markedly at 21 days, and returned to levels comparable to controls by 28 days. This transient expansion coincided with the late-stage activation of CD4+ and CD8+ T cells. The authors note that increased Treg numbers can suppress tissue-damaging inflammation but may also promote pathogen replication and persistence, and they report that this is the first demonstration of PCV2 infection inducing Treg expansion in C57BL/6 mice, providing new evidence for the immunosuppressive mechanisms underlying PCV2 pathogenesis.

The authors caution that the immune response pattern induced by PCV2d may not be entirely unique. Although amino acid variations exist in the Cap proteins among PCV2a, PCV2b, and PCV2d, research has confirmed that these genotypes can induce cross-protective immunity, and commercial PCV2a vaccines have been shown to protect against both PCV2b and PCV2d. Inactivated vaccines of all three genotypes effectively reduce viral loads, indicating comparable protective immunity. However, recent single-cell transcriptomic analyses have revealed that PCV2d infection triggers a unique immune response in murine lung tissue, and the authors call for further investigations to determine whether the immune signature they observed is specific to PCV2d or represents a common feature across different PCV2 genotypes. The work was performed with at least three independent replicates per experiment, analyzed with two-tailed Student’s t tests in GraphPad Prism, and the authors state that the underlying data are available from the corresponding author upon reasonable request.

Subject of Research: Immune responses to porcine circovirus type 2d infection in mice

Article Title: Host immune responses to porcine circovirus type 2d infection in C57BL/6 mice: lymphocyte activation, thymic T-cell dynamics, and Treg expansion

Article References: Qiu, Y., Liu, J., Yang, X., Hou, L., Lan, X., Zhou, B., Zhang, Q., Gao, W., Dong, X., Shang, S., Zhou, J., & Liu, J. (2026). Host immune responses to porcine circovirus type 2d infection in C57BL/6 mice: lymphocyte activation, thymic T-cell dynamics, and Treg expansion. Animal Diseases, 6(1), Article 41. https://doi.org/10.1186/s44149-026-00252-8

Image Credits: AI Generated

DOI: 10.1186/s44149-026-00252-8

Keywords: PCV2d, porcine circovirus type 2, C57BL/6 mice, lymphocyte activation, thymic apoptosis, Treg cells, PD-1, T-cell exhaustion, flow cytometry, swine health, viral immunology, memory T cells