Multiple sclerosis lesion activity concurs with the extent of inflammation, demyelination and axonal suffering. Pro-inflammatory myeloid cells contribute to lesion development, but the self-limiting nature of lesions implies as yet unidentified anti-inflammatory mechanisms. We addressed the hypothesis that myelin ingestion by myeloid cells induces a foamy appearance and confers anti-inflammatory function. First, we show that myelin-containing foam cells in multiple sclerosis lesions consistently express a series of anti-inflammatory molecules while lacking pro-inflammatory cytokines. Second, unique location-dependent cytokine and membrane receptor expression profiles imply functional specialization allowing for differential responses to micro-environmental cues. A novel human in vitro model of foamy macrophages functionally confirmed that myelin ingestion induces an anti-inflammatory programme. Foamy macrophages are unable to respond to prototypical inflammatory stimuli but do express molecules involved in suppression of inflammation. These findings provide novel insights into the mechanisms of lesion control and may open new roads to intervention.

Despite lack of classical lymphatic vessels in the central nervous system (CNS), cells and antigens do reach the CNS-draining lymph nodes. These lymph nodes are specialized to mediate mucosal immune tolerance, but can also generate T- and B-cell immunity. Their role in multiple sclerosis and experimental autoimmune encephalomyelitis (EAE) therefore remains elusive. We hypothesized that drainage of CNS antigens to the CNS-draining lymph nodes is vital for the recurrent episodes of CNS inflammation. To test this, we surgically removed the superficial cervical lymph nodes, deep cervical lymph nodes, and the lumbar lymph nodes prior to disease induction in three mouse EAE models, representing acute, chronic, and chronic-relapsing EAE. Excision of the CNS-draining lymph nodes in chronic-relapsing EAE reduced and delayed the relapse burden and EAE pathology within the spinal cord, which suggests initiation of CNS antigen-specific immune responses within the CNS-draining lymph nodes. Indeed, superficial cervical lymph nodes from EAE-affected mice demonstrated proliferation against the immunizing peptide, and the deep cervical lymph nodes, lumbar lymph nodes, and spleen demonstrated additional proliferation against other myelin antigen epitopes. This indicates that intermolecular epitope spreading occurs and that CNS antigen-specific immune responses are differentially generated within the different CNS-draining lymphoid organs. Proliferation of splenocytes from lymphadenectomized and sham-operated mice against the immunizing peptide was similar. These data suggest a role for CNS-draining lymph nodes in the induction of detrimental immune responses in EAE relapses, and conclusively demonstrate that the tolerance-inducing capability of cervical lymph nodes is not involved in EAE.

Drainage of central nervous system (CNS) antigens to the brain-draining cervical lymph nodes (CLN) is likely crucial in the initiation and control of autoimmune responses during multiple sclerosis (MS). We demonstrate neuronal antigens within CLN of MS patients. In monkeys and mice with experimental autoimmune encephalomyelitis (EAE) and in mouse models with non-inflammatory CNS damage, the type and extent of CNS damage was associated with the frequencies of CNS antigens within the cervical lymph nodes. In addition, CNS antigens drained to the spinal-cord-draining lumbar lymph nodes. In human MS CLN, neuronal antigens were present in pro-inflammatory antigen-presenting cells (APC), whereas the majority of myelin-containing cells were anti-inflammatory. This may reflect a different origin of the cells or different drainage mechanisms. Indeed, neuronal antigen-containing cells in J Mol Med (2009) human CLN did not express the lymph node homing receptor CCR7, whereas myelin antigen-containing cells in situ and in vitro did. Nevertheless, CLN from EAE-affected CCR7-deficient mice contained equal amounts of myelin and neuronal antigens as wild-type mice. We conclude that the type and frequencies of CNS antigens within the CLN are determined by the type and extent of CNS damage. Furthermore, the presence of myelin and neuronal antigens in functionally distinct APC populations within MS CLN suggests that differential immune responses can be evoked.