Background: Pregnancy induces alterations in the maternal immune system. Multiple sclerosis (MS) is an inflammatory autoimmune disease of the central nervous system (CNS), which is ameliorated during pregnancy. The exact mechanisms leading to this are poorly understood, but it is likely that the immunological changes which contribute to a successful pregnancy also help control autoimmunity. Pregnancy-related alterations may also affect the immunosurveillance of opportunistic pathogens, which can render the mother susceptible to certain infections such as listeriosis and JCV-induced PML. 

Aim: To investigate immunological mechanisms contributing to amelioration of MS during pregnancy. To evaluate how pregnancy-related immunological alterations affect the susceptibility to an opportunistic infection particularly relevant in care of MS, namely JCV-induced PML. 

Methods: Blood and cerebrospinal fluid (CSF) samples and clinical data were collected longitudinally during pregnancy and 6 months postpartum from MS-patients participating in a nationwide Finnish Multiple Sclerosis and Pregnancy Study. Alterations in peripheral blood lymphocyte subpopulations, MS-associated CSF determinants (IgG-index, number of oligoclonal bands and lymphocytes) and humoral responses to JCV were evaluated during and after pregnancy. 

Results: MS-pregnancy is characterized by non-altered proportions of T- and B-cells, increased proportion of regulatory CD56bright NK-cells, decreased proportions of CD16+ and CD3-CD56dim NK-cells, and increased Th2/Th1 cytokine ratio. The CSF IgG-Index was significantly higher during pregnancy than postpartum. JCV-Ab-indices increased postpartum. 

Conclusions: Regulatory CD56bright NK-cells might facilitate the control of autoimmune inflammation during pregnancy and thus contribute to the reduced MS activity during pregnancy. Hightened postpartum Ab-responses to JCV likely reflect altered immunity towards JCV during pregnancy.