Hypotheses that could explain this phenomenon:

 

1. Impairment or enhancement of some cytokines. Impairment of some cytokines such as IFN-α or TNF-α can affect the development of an effective Th1 immune response. Also, the induction of IL-10 by PRRS virus may negatively regulate the cell responses.
   
    
2. Interference with correct antigen presentation and activation of T lymphocytes. The PRRS virus down-regulation of given cytokines or of immunologically relevant cell-surface molecules during antigen presentation can affect the development of effective cell responses.
   
    
3. Presence of T regulatory (Treg) cells. As seen before, the major function of Treg cells is to down-regulate immune responses. In PRRS virus-infected pigs, a positive correlation has been demonstrated between viraemia and induced Treg cells. These cells have been consistently observed in lymphoid tissues after infection and have been related to TGF-β production. Differences with regards to the ability to generate Tregs could exist among PRRS virus isolates.

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