But I found an interesting article regarding the different types of MS (RRMS and PPMS) and the reduction in thymic output and also a mention of immune system rebooting:
Direct analysis of thymic export suggests that thymic alterations occur early in life in RRMS and PPMS, either at the time of MS onset or before and that the thymic ‘defect’ persists throughout life, as indicated by low but constant levels of thymic export with increasing age. In other words, our findings suggest an early-onset of thymic involution in MS.
One study of immunoablation/autologous stem cell transplantation as a treatment of MS showed that the posttransplant MS patients ‘rebooted’ their immune system such that the thymus produced new Tcells having a diverse TCR repertoire . This finding is not inconsistent with our results: RRMS over age 40 continue to generates recent thymic emigrants, which contain the majority of naïve T cell TCR diversity , although this generation is at significantly lower levels than age matched
I think there may be an age component between RRMS and PPMS such that the thymus may be more deficient in creating healthy T-cells as one ages (or progresses in disease). It seems analogous to people that are struck with rheumatoid arthritis as they age.
This makes me think that maybe this has an influence of the effectiveness of HCST on PPMS patients.