In aged mice, low surrogate light chain promotes pro-B-cell apoptotic resistance, compromises the PreBCR checkpoint, and favors generation of autoreactive, phosphorylcholine-specific B cells

In aged mice, new B-cell development is diminished and the antibody repertoire becomes more autoreactive. Our studies suggest that (i) apoptosis contributes to reduced B lymphopoiesis in old age and preferentially eliminates those B-cell precursors with higher levels of the surrogate light chain (SLC) proteins (λ5/VpreB) and (ii) λ5(low) B-cell precursors generate new B cells which show increased reactivity to the self-antigen/bacterial antigen phosphorylcholine (PC). Pro-B cells in old bone marrow as well as pro-B cells from young adult λ5-deficient mice are resistant to cytokine-induced apoptosis (TNFα; TGFβ), indicating that low λ5 expression in pro-B cells is sufficient to cause increased survival. Transfer of TNFα-producing 'age-associated B cells' (ABC; CD21/35(-) CD23(-)) or follicular (FO) B cells from aged mice into RAG-2 KO recipients led to preferential loss of λ5(high) pro-B cells, but retention of λ5(low), apoptosis-resistant pro-B cells. In old mice, there is increased reactivity to PC in both immature bone marrow B cells and mature splenic FO B cells. In young mice, absence of λ5 expression led to a similar increase in PC reactivity among bone marrow and splenic B cells. We propose that in old age, increased apoptosis, mediated in part by TNFα-producing B cells, results in preferential loss of SLC(high) pro-B cells within the bone marrow. Further B-cell development then occurs via an 'SLC(low)' pathway that not only impairs B-cell generation, but promotes autoreactivity within the naïve antibody repertoires in the bone marrow and periphery.

Low surrogate light chain in pro-B cells from old mice suppresses apoptosis and alters the idiotypes and proportions of phosphorylcholine reactive B cells (LYM6P.718)

The generation of new B cells is substantially decreased in old mice (>2 years). Early B cell precursors (pro-B cells) are reduced in old mice and exhibit a reduction in the surrogate light chain (SLC) protein λ5. We hypothesize that this alters the expression and function of the pre-B cell receptor and influences the “read-out” of antibody specificities in the B cell repertoire. In old mice, the remaining pro-B cells, low in SLC, are resistant to apoptotic stimuli (TNFα), in contrast to young adult pro-B cells. Apoptotic sensitivity reflects levels of the SLC, since young pro-B cells partially or completely deficient in λ5 are also resistant to apoptosis. Age-associated B cells (ABC) represent a B cell subset which expands in old mice; ABC are pro-inflammatory and secrete TNFα. Adoptive transfer of ABC into young RAG-2 KO recipients results in partial loss of pro-B cells and, in particular, loss of pro-B cells with higher levels of SLC λ5. Anti-phosphorylcholine (PC) reactive B cells, especially those of the T15 idiotype, are important in defense against pneumococci. In old age, the proportion of PC reactive B cells, particularly with less protective T15 negative idiotypes, increases. Low SLC promotes both increased PC reactivity and T15 negative idiotypes. These studies suggest that ABC promote an “SLClow” B cell pathway that alters antibody specificities in old age.

MRI in unexplained mononeuropathy

Four young patients with severe unexplained progressive mononeuropathy are described. None had a history of known trauma to the affected limb. In addition to the standard neurologic examination and electrophysiologic studies (nerve conduction studies and electromyography), all underwent neuroimaging of the involved extremity. In three patients, magnetic resonance imaging revealed intrinsic abnormalities of the appropriate nerve. The pattern or absence of magnetic resonance imaging changes directly influenced decisions about surgical exploration of the nerve in all four patients. With the advent of more sophisticated technology, magnetic resonance neurography has become a potent diagnostic tool in the evaluation of disorders of peripheral nerve and muscle.

Fulminating encephalopathy with perivenular demyelination and vacuolar myelopathy as the initial presentation of human immunodeficiency virus infection

OBJECTIVE

To study the neuropathologic features in a case involving a 22-year-old woman in whom a fulminating encephalopathy developed as the initial manifestation of human immunodeficiency virus (HIV) infection.

DESIGN

Case report.

SETTING

Tertiary care hospital.

PATIENT

The patient presented with rapidly progressive mental status, changes, cranial nerve abnormalities, and quadriplegia, which led to her death 5 months later. Serologic tests for HIV were initially indeterminate on Western blot analysis but were positive 1 week later.

METHODS

A complete autopsy, including examination of the brain and spinal cord, was performed. Paraffin-embedded sections of the brain and spinal cord were examined using standard histologic staining procedures and immunohistochemical techniques.

RESULTS

Neuropathologic examination revealed discrete foci of perivenular demyelination disseminated throughout the brain and spinal cord, as well as severe vacuolar myelopathy. Lesions typical of HIV encephalitis were not present. Human immunodeficiency virus-infected monocytes and microglia were observed in the vicinity of, but not restricted to, the perivenular demyelinating lesions. No other infectious agents were identified.

CONCLUSIONS

The patient's acute encephalopathy was most likely the direct result of a widespread demyelinating process resembling acute disseminated encephalomyelitis. We suggest that the perivenular demyelination may represent an autoimmune reaction, possibly due to a nonspecific viral infection, occurring in the setting of chronic immunosuppression secondary to HIV. Although less likely, we cannot exclude the possibility that HIV could have directly triggered an autoimmune response that caused the acute disseminated encephalomyelitis-like lesions.