Estradiol levels, which are low during the first half of the menstrual cycle, rise and peak 2–3 days before ovulation.
During ovulation, humoral and cell-mediated immune responses are believed to be suppressed in order to provide optimal conditions for procreation.
However, in the normal menstrual cycle, this may be related to a ‘window of vulnerability’ during the menstrual cycle beginning at ovulation and lasting 7–10 days. In this period the potential for viral infectivity in the female reproductive tract (FRT) is enhanced.
During that period, aspects of the innate, humoral, and cell-mediated immune systems are suppressed by sex hormones, and, thus, females are more susceptible to infections of the FRT, including Candida albicans, genital herpes simplex virus, Chlamydia trachomatis and HIV.
nearly all pregnancies can be attributed to intercourse during a six-day period ending on the day of ovulation.
Thus, the above-mentioned time-frame is also considered to be causally related to an ‘immune tolerance’ to the allogeneic sperm, and allows semen to survive during ovulation and during this ‘window of opportunity and tolerance’ to increase the chance of fertilization. This phenomenon and the overall hormonal control of FRT’s infections and reproductive functions remain unclear.
Dendritic cells(DCs) link innate immunity to adaptive immunity. Upon exposure to microbial invaders in tissue, they undergo a maturational process that culminates in the lymph nodes and activates T-cell-specific immune responses. Estradiol, which is highly expressed during ovulation, has an effect on the maturation of DCs, although the molecular mechanism remains elusive.
The biological actions of estrogen are mediated through association of the E2 receptor proteins with the estrogen-responsive elements (EREs) in the promoters of E2-regulated genes.
In the Genes & Immunity study S. Lasarte et al. from the Hospital General Universitario Gregorio Marañón, Madrid, Spain to further investigate the molecular mechanism by which E2 downregulates DC functions analyzed the expression of genes whose regulatory regions contain EREs, and provide new insights into these mechanisms.
The authors of this study demonstrate that estradiol is able to suppress the NF-κb translocation to the nucleus, a step crucial for the dendritic cells maturation process, by its transcriptional downregulation of Ikbkg expression at the promoter level in DCs.
Ikbkg is the regulatory subunit of the inhibitor of NF-kb kinase (IKK) complex. Nf-kb dimers remain inactive in the cytoplasm by association with Nfkbia. After stimulation, the IKK complex phosphorylates Nfkbia and undergoes proteasomal degradation.
Free Nf-kb can then translocate into the nucleus and stimulate the transcription of target genes. This translocation step is necessary for the promotion of the DC maturation program that induces migration and ends with the antigen presentation to the T cells in the lymph nodes.
The investigators found that detected estradiol regulates expression of Ikbkg in DCs and modulates nuclear factor-κb translocation to the nucleus. This change may be an adaptive mechanism to reconcile control of infection and reproductive functions.
This may explain the reduced DCs’ function observed during ovulation. The authors suggest that estradiol–inhibited DCs would not trigger the maturation program, migrate to the T-cell areas and upregulate antigen presentation proteins during estrus (high estradiol) so that spermatozoa can survive.
Therefore, allogeneic spermatozoa could reach and be stored at the oviduct in order to acquire the ability to fertilize. According to the authors
DCs are ‘shutdown’ due to this hormone-driven adaptive mechanism that may bring together the female immune response with reproductive function.