Although the brain is considered to be iso-lated from the peripheral immune system, ithas been shown that astrocytes and microgliacan express innate immune molecules, in-cluding all components of the complementsystem, sufficient to generate a local anti-microbial defense system. Here, Gasque andcolleagues show that the serine protease per-forin can be synthesized by fetal astrocytes inculture, as well as by adult astrocytes duringinflammation. This is the first demonstrationof perforin in non-lymphoid cells and sug-gests that a more-complex innate immunedefense strategy is possible within the brain.

Reverse transcriptase polymerase chainreaction (RT-PCR) derived a fetal astrocyteperforin cDNA sequence that was identicalto the reported perforin cDNA cloned fromT cells. Western blot analysis using mono-clonal and polyclonal antiperforin peptideantibodies revealed a protein of 65 kDa inhuman fetal astrocytes matching that incontrol lysates from rat natural killer (NK)cells. Immunofluorescence and confocal

microscopy confirmed that perforin was ex-pressed by 40–50% of glial fibrillary acidicprotein (GFAP)-positive cells present in thefetal brain culture, so detection was not due to contaminating neural fibroblasts(GFAP-negative). Interestingly, in contrastto lymphoid cells, astrocyte perforin wasnot localized to granules but was presentthroughout the cytoplasm, probably in as-sociation with the endoplasmic reticulum.Perforin was not detected in normal adultbrain tissue but was present in and aroundareas of inflammation (white and grey matter), both in multiple sclerosis andneurodegenerative brains. These perforin-positive cells in inflammatory foci wereidentified as reactive astrocytes.

It must be noted that the study was notable to demonstrate lytically active per-forin. However, these findings demonstratethat perforin expression is not unique tolymphoid cells and suggest that perforinproduced by a subpopulation of astrocytesplays a role in inflammation in the brain.

B-cell migration to the B-cell areas of second-ary follicles is essential for germinal centre(GC) formation prior to production of highaffinity immunoglobulin and developmentof B-cell memory. Although chemokines arethought to direct B-cell migration into sec-ondary follicles, a B-cell specific chemokinehas not been described until now. Here,Gunn and colleagues report on the isolationof a mouse B-lymphocyte chemoattractant(BLC), a potential ligand for the previouslydescribed B-cell chemokine receptor Burkitt’slymphoma receptor 1 (BLR-1).

A genomic approach was used to screenexpressed sequence tag (EST) databases forhomology to human monocyte chemo-attractant protein 1 (MCP-1). Four cloneswere retrieved, of which one (IMAGE Consortium clone 596050) was shown tobind to B-cell rich areas of splenic folliclesand Peyer’s patches using an antisense insitu hybridization strategy. Full lengthcDNA was shown to encode a putative protein of 109 amino acids, with the four-cysteine pattern typical of the CXC familyof chemokines: the closest homology waswith GROa. To assess chemokine activity,baculovirus-expressed recombinant BLCwas used in transwell-migration studies onlymphocytes and BLR-1-transfected Jurkatcells. Only B cells or BLR-1-transfected cellsresponded to BLC, whereas T cells, granu-locytes and macrophages did not. Further-more, BLC stimulated Ca2+ fluxes only in Bcells and BLR-1-transfected HEK cells. HEKtransfectants expressing CCR1, CCR2 orCXCR2 were not responsive to BLC.

The discovery of BLC, a novel, B-cellspecific CXC chemokine, provides at leastone clear mechanism by which B cells ex-pressing BLR-1 may be attracted to the B-cell areas of secondary lymphoid tissues.The source of BLC during this process is notknown, however, the presence of folliculardendritic cells (FDCs) in these restrictedareas suggests that FDCs are a strong candidate for BLC synthesis.

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Current literature

BLC: the emergence of a B-cellchemokineGunn, M.D., Ngo, V.N., Ansel, K.M. et al.(1998) A B-cell-homing chemokine made inlymphoid follicles activates Burkitt’slymphoma receptor 1 Nature 391, 799–803

The T-cell receptor (TCR) must recognize self-antigens in the thymus during maturationand then discriminate between foreign patho-gens in the periphery. This pattern of reactiv-ity implies a degree of plasticity and accom-modation in the binding event with majorhistocompatibility complex (MHC) moleculesand their bound peptide. Garcia and col-leagues suggest a molecular basis for thiscrossreactivity in this study on the well-char-acterized 2C TCR, which is specific for self H-2Kb bound to a peptide designated dEV8,but is crossreactive with H-2Ld in associationwith a mitochondrial self-peptide p2Ca.

The 3.0 Å crystal structure of 2C TCRbound to H-2Kb–dEV8 shows that the inter-face between peptide and TCR exhibits ex-tremely poor shape complementarity, and theTCR b-chain complementarity-determiningregion 3 (CDR3) has minimal interaction with

the dEV8 peptide. Large conformationalchanges in three of the TCR CDR loops areinduced upon binding, confirming that struc-tural plasticity is a means by which recogni-tion of a variety of peptide–MHC complexescan be accommodated. Extensive TCR inter-action with the MHC a-helices suggests ageneralized orientation that is mediated bythe Va domain of the TCR. There is sufficientspace between the peptide and the TCR to accommodate a potentially wide range ofpeptide residues.

These data all indicate that the TCR–peptide interface is not one of ‘exquisite’specificity and TCRs can thus ‘scan’ differentpeptides bound within a large, low-affinityMHC structural framework for those thatprovide the slight additional kinetic stabiliz-ation required for signalling during positiveor negative selection.

‘Scanning’ recognition by TCRsGarcia, K.C., Degano, M., Pease, L.R. et al. (1998) Structural basis of plasticity in T-cell receptor recognition of a self peptide–MHC antigen Science 279, 1166–1172

Brain damage: the case for astrocyte-derived perforinGasque, P., Jones, J., Singhrao, S.K. and Morgan, B.P. (1998) Identification of an astrocyte cellpopulation from human brain that expresses perforin, a cytotoxic protein implicated inimmune defense J. Exp. Med. 187, 451–460