豬隻周邊血液單核球分化程度對第二型豬環狀病毒感受性之影響

Abstract

第二型豬環狀病毒(porcine circovirus type II, PCV2)為豬隻離乳後多系統消耗性症候群(post-weaning multisystemic wasting syndrome, PMWS)之主要病原。目前，已知免疫系統內的單核球/巨噬細胞此一系列的細胞是PCV2之主要標的細胞之一。然而，在許多in vitro試驗中卻無法證實PCV2可於此類細胞內進行複製。先前本實驗室研究結果發現LPS可促使PCV2從豬肺泡巨噬細胞的細胞質轉移到細胞核內，並且在細胞核內進行複製與增殖。由於組織中成熟的巨噬細胞是經由周邊血液中較年輕且未成熟的單核球分化而成，而細胞的分化可能會誘導一些對病毒複製而言必需的因子，像是接受器或者是轉錄因子，而細胞的活化可調節這些因子在細胞內的升降趨勢，本實驗的目的是為了進ㄧ步瞭解周邊血液單核球在有無LPS刺激下，其細胞分化程度對PCV2感受性的影響。本實驗研究結果顯示健康PCV2帶原豬隻周邊血液單核球經過長時間的離體培養後，可讓少量的PCV2抗原從細胞質轉移到細胞核內，並且在該等細胞內進行PCV2的複製與增殖。反之，若給予適當的LPS刺激後，能促使PCV2抗原從細胞質提早進入細胞核內，並且在該等細胞內進行PCV2的複製與增殖。此外，不論有無LPS刺激下，豬隻周邊血液單核球對PCV2的感受性皆會隨細胞分化程度的提升而有增加的趨勢；然而，經過LPS刺激之細胞似乎有稍微提早成熟的趨勢，但此作用並不明顯。因此，推測LPS可能經由某些特殊傳遞途徑進而活化細胞並促使PCV2提早進入細胞核內進行複製，而細胞分化促使PCV2複製之確實機制，則需要再做進一步的探討。

Porcine circovirus type 2 (PCV2) is the primary causative agent of postweaning multisystemic wasting syndrome (PMWS). At present, monocyte/macrophage lineage cells are considered as one of the major target cells; however, no evidence of PCV2 replication has been demonstrated in these cells in vitro. Our previous studies have led to the speculation that PCV2 once enters porcine alveolar macrophages (PAMs) by either passive endocytosis or phagocytosis may start intranuclear translocation and proliferation following lipopolysaccharide (LPS) stimulation. Tissue macrophages are derived from blood monocytes. Cell differentiation may induce some factors which may enhance viral infection and replication, such as receptors or transcription factors, and cell activation may up- or down-regulate the expression of these factors. Thus, the objective of the study was to evaluate the effect of cell differentiation with or without LPS stimulation on the susceptibility of peripheral blood monocytes (PBMs) to PCV2 infection. The results showed that a small amount of PCV2 antigens could be translocated from the cytoplasm into nucleus followed by viral replication with time in PBMs collected from healthy PCV2-carrier pigs after long term in vitro culture. Stimulation with appropriate concentration of LPS could promote earlier intranuclear translocation of PCV2 antigens and proliferation of PCV2 in PBMs. Moreover, the susceptibility of PBMs to PCV2 increased as PBMs transformed into macrophages with or without LPS stimulation; however, LPS stimulation seemed to induce a relatively quicker but insignificant maturation in PBMs. It is speculated that LPS may activate cells and promote intranuclear translocation of PCV2 earlier through some specific signal transduction pathways, and the actual mechanism of cell differentiation in promoting PCV2 replication needs to be further elucidated.